Okay. I'm getting "The remote server returned an error: (500)
internal server error".

I've got a form with two text boxes in the upper left corner,
txtHalfID and txtHalfPassword. There are three text boxes in the
upper right corner, txtISBN, txtDescription and txtPrice. There's
also a drop down combo in the upper right, cmbCondition.

The bottom portion of the screen has a read only multi line text
box called txtResponse, where the raw HTML from the server is
displayed.

Below the left text boxes is a button, btnSignOn. If you click
that button, it will sign you onto Half.com using the ID and
password in the textboxes above it. This part works fine for me.
I can see the "Welcome to eBay" page in txtResponse.

Below the right boxes is a button called btnListBook. When I
click on it it should call four web pages on half.com in order.

First it calls http://half.ebay.com/help/sell_books.cfm. It fills
in the field in the form there with txtISBN.

Next it calls http://half.ebay.com/cat/sell/pmsearch.cgi. It
fills out the form there with txtDescription and cmbCondition.
This form allows file uploads, so it's a multipart form. Although
my program's output looks good to me, I get that server error
when I submit this form.

Here's the code:

- Start Code ---------------------------------------------------
Imports System.Net

Public Class SBM
   Inherits System.Windows.Forms.Form

   Dim cc As New CookieCollection
   Const encURL As Integer = 0
   Const encMulti As Integer = 1

   Private Sub SBM_Load(ByVal sender As System.Object, ByVal e
As System.EventArgs) Handles MyBase.Load
       txtHalfID.Text = ""
       txtHalfPassword.Text = ""

       txtISBN.Text = "0764560255"
       txtDescription.Text = "good good"
       txtPrice.Text = "99.99"

       With cmbCondition
           .Items.Add(New Conditions("Brand New", "830"))
           .Items.Add(New Conditions("Like New", "840"))
           .Items.Add(New Conditions("Very Good", "849"))
           .Items.Add(New Conditions("Good", "859"))
           .Items.Add(New Conditions("Acceptable", "864"))
           .SelectedIndex = 0
       End With

       cmbCondition.SelectedIndex = 0
   End Sub

   Private Sub btnSignOn_Click(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles btnSignOn.Click
       Dim HWRequest As HttpWebRequest
       Dim strURL As String
       Dim HWParameters As Parameters
       Dim intFound As Integer

       HWRequest = GetRequest
("https://signin.ebay.com/ws/eBayISAPI.dll?SignIn&UsingSSL=1
&co_partnerid=2&siteid=20")
       strURL = "https://signin.half.ebay.com/ws/eBayISAPI.dll"

       HWParameters = ReadResponse(HWRequest, strURL)

       intFound = 0

       For Each p As Parameter In HWParameters
           Select Case p.Name
               Case "userid"
                   p.Value = txtHalfID.Text
                   intFound += 1
               Case "pass"
                   p.Value = txtHalfPassword.Text
                   intFound += 1
           End Select

           If intFound = 2 Then
               Exit For
           End If
       Next

       HWRequest = PostRequest(strURL, HWParameters, encURL)
       GetResponse(HWRequest)
   End Sub

   Private Sub btnListBook_Click(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles btnListBook.Click
       Dim strPostData As String
       Dim HWRequest As HttpWebRequest
       Dim strURL As String
       Dim HWParameters As Parameters
       Dim intFound As Integer
       Dim pr As Parameter

       Try
           HWRequest = GetRequest
("http://half.ebay.com/help/sell_books.cfm")
           strURL = "http://half.ebay.com/cat/sell/pmsearch.cgi"

           HWParameters = ReadResponse(HWRequest, strURL)

           intFound = 0

           For Each p As Parameter In HWParameters
               Select Case p.Name
                   Case "p_code"
                       p.Value = txtISBN.Text
                       intFound += 1
               End Select

               If intFound = 1 Then
                   Exit For
               End If
           Next

           HWRequest = PostRequest(strURL, HWParameters, encURL)

           strURL = "/cat/sell/save_new_listing.cgi"

           HWParameters = ReadResponse(HWRequest, strURL)
           pr = New Parameter
           pr.Name = "x"
           pr.Value = 20
           pr.Type = Parameter.Input
           HWParameters.Add(pr)
           pr = New Parameter
           pr.Name = "y"
           pr.Value = 20
           pr.Type = Parameter.Input
           HWParameters.Add(pr)
           intFound = 0

           For Each p As Parameter In HWParameters
               Select Case p.Name
                   Case "notes"
                       p.Value = txtDescription.Text
                       intFound += 1
                   Case "condition"
                       p.Value = cmbCondition.Items
(cmbCondition.SelectedIndex).ItemData
                       intFound += 1
               End Select

               If intFound = 2 Then
                   Exit For
               End If
           Next

           strURL =
"http://half.ebay.com/cat/sell/save_new_listing.cgi"

           HWRequest = PostRequest(strURL, HWParameters,
encMulti)

           strURL = "sell.jsp"
           HWRequest.Referer =
"http://half.ebay.com/cat/sell/pmsearch.cgi"

'the internal server error is generated in this call
'to ReadResponse
           HWParameters = ReadResponse(HWRequest, strURL)
           intFound = 0

           For Each p As Parameter In HWParameters
               Select Case p.Name
                   Case "itemPrice"
                       p.Value = txtPrice.Text
                       intFound += 1
               End Select

               If intFound = 1 Then
                   Exit For
               End If
           Next

           strURL = "http://half.ebay.com/cat/sell/sell.jsp"

           HWRequest = PostRequest(strURL, HWParameters, encURL)

           GetResponse(HWRequest)

           MessageBox.Show("Success!", MsgBoxStyle.OKOnly, "Book
Posted")

       Catch ex As Exception
           MessageBox.Show("Error: " & ex.Message, "Error!!!",
MessageBoxButtons.OK, MessageBoxIcon.Exclamation)

           MsgBox("Half.com appears to have changed its posting
procedures. As a result, this version of SBM cannot list books on
it.", MsgBoxStyle.Exclamation, "Half.com Listing Problem")
       End Try
   End Sub

   Private Function GetRequest(ByVal strURL As String) As
HttpWebRequest
       GetRequest = CreateRequest(strURL)
       GetRequest.Method = "GET"
   End Function

   Private Function PostRequest(ByVal strURL As String, ByVal
PostParameters As Parameters, ByVal intType As Integer) As
HttpWebRequest
       Dim encoding As New System.Text.ASCIIEncoding
       Dim byte1 As Byte()
       Dim newStream As System.IO.Stream
       Dim strData As String
       Const strBoundary As String =
"---------------------------7d4285126106b0"

       PostRequest = CreateRequest(strURL)
       PostRequest.Method = "POST"

       If intType = encURL Then
           PostRequest.ContentType = "application/x-www-form-
urlencoded"
       Else
           PostRequest.ContentType = "multipart/form-data,
boundary=" & strBoundary
       End If

       strData = BuildRequestString(PostParameters, intType,
strBoundary)

       PostRequest.ContentLength = strData.Length

       byte1 = encoding.GetBytes(strData)

       newStream = PostRequest.GetRequestStream
       newStream.Write(byte1, 0, byte1.Length)
       newStream.Close()
   End Function

   Private Function BuildRequestString(ByVal RequestParameters
As Parameters, ByVal intType As Integer, ByVal strBoundary As
String)
       BuildRequestString = ""

       Select Case intType
           Case encURL
               For Each p As Parameter In RequestParameters
                   BuildRequestString &= p.Name.Trim & "=" &
p.Value.Trim & "&"
               Next

               BuildRequestString = Mid(BuildRequestString, 1,
Len(BuildRequestString) - 1)
           Case encMulti

'the requeststring generated for the multipart form is
'shown below

               For Each p As Parameter In RequestParameters
                   BuildRequestString &= "--" & strBoundary &
vbCrLf & "Content-Disposition: form-data; name=""" & p.Name.Trim
& """"
                   Select Case p.Type
                       Case Parameter.File
                           BuildRequestString &= ";
filename=""""" & vbCrLf & "Content-Type: application/octet-
stream" & vbCrLf & vbCrLf & vbCrLf
                       Case Parameter.Input
                           BuildRequestString &= vbCrLf & vbCrLf
& p.Value & vbCrLf
                   End Select
               Next
               BuildRequestString &= "--" & strBoundary & "--"
       End Select
   End Function

   Private Function CreateRequest(ByVal strURL As String) As
HttpWebRequest
       Dim Uri As Uri

       Uri = New Uri(strURL)
       CreateRequest = HttpWebRequest.Create(Uri)
       CreateRequest.AllowAutoRedirect = True
       CreateRequest.CookieContainer = New CookieContainer

       If cc.Count > 0 Then
           CreateRequest.CookieContainer.Add(cc)
       End If
   End Function

   Private Sub GetResponse(ByVal ReadRequest As HttpWebRequest)
       Dim HWResponse As HttpWebResponse
       Dim cookie As Cookie

       HWResponse = ReadRequest.GetResponse()

       If HWResponse.Cookies.Count > 0 Then
           For Each cookie In HWResponse.Cookies
               cc.Add(cookie)
           Next
       End If

       'debugging
       Dim sr As System.IO.StreamReader
       Dim strResult As String

       sr = New System.IO.StreamReader
(HWResponse.GetResponseStream())

       txtResponse.Text = sr.ReadToEnd

       sr.Close()
       'debugging
   End Sub

   Private Function ReadResponse(ByVal ReadRequest As
HttpWebRequest, ByVal ReadURL As String) As Parameters
       Dim HWResponse As HttpWebResponse
       Dim cookie As Cookie
       Dim sr As System.IO.StreamReader
       Dim strResult As String
       Dim intTagPos As Integer
       Dim intLength As Integer
       Dim intURLPos As Integer
       Dim strTag As String
       Dim blnFormFound As Boolean
       Dim blnMoreForms As Boolean
       Dim intPos As Integer
       Dim strName As String
       Dim strValue As String
       Dim ReadParameter As Parameter
       Dim intType As Integer
       Dim intInputPos As Integer
       Dim intTextPos As Integer
       Dim intSelectPos As Integer
       Dim blnTagFound As Boolean

       ReadResponse = New Parameters

'the next line generates the internal server error
       HWResponse = ReadRequest.GetResponse()

       If HWResponse.Cookies.Count > 0 Then
           For Each cookie In HWResponse.Cookies
               cc.Add(cookie)
           Next
       End If

       sr = New System.IO.StreamReader
(HWResponse.GetResponseStream())

       strResult = sr.ReadToEnd

       'debugging
       txtResponse.Text = strResult
       'debugging

       blnFormFound = False
       blnMoreForms = True
       intTagPos = 1

       While Not blnFormFound And blnMoreForms
           intTagPos = InStr(intTagPos, strResult.ToUpper,
"<FORM", CompareMethod.Text)

           If intTagPos <> 0 Then
               intLength = InStr(intTagPos, strResult, ">",
CompareMethod.Text) - intTagPos
               strTag = strResult.Substring(intTagPos,
intLength)

               intURLPos = InStr(1, strTag.ToUpper,
ReadURL.ToUpper, CompareMethod.Text)

               If intURLPos <> 0 Then
                   intLength = InStr(intTagPos,
strResult.ToUpper, "</FORM>", CompareMethod.Text) - intTagPos
                   strResult = strResult.Substring(intTagPos,
intLength)
                   blnFormFound = True
               Else
                   intTagPos += 1
               End If
           Else
               blnMoreForms = False
           End If
       End While

       If blnFormFound Then
           intTagPos = 1

           While intTagPos <> 0
               intInputPos = InStr(intTagPos, strResult.ToUpper,
"<INPUT", CompareMethod.Text)
               intTextPos = InStr(intTagPos, strResult.ToUpper,
"<TEXTAREA", CompareMethod.Text)
               intSelectPos = InStr(intTagPos,
strResult.ToUpper, "<SELECT", CompareMethod.Text)

               intTagPos = IIf(intTextPos <> 0 And intTextPos <
intInputPos, intTextPos, intInputPos)
               intTagPos = IIf(intSelectPos <> 0 And
intSelectPos < intTagPos, intSelectPos, intTagPos)

               intType = Parameter.Input

               If intTagPos <> 0 Then
                   intLength = InStr(intTagPos, strResult, ">",
CompareMethod.Text) - intTagPos
                   strTag = strResult.Substring(intTagPos,
intLength)

                   intPos = InStr(1, strTag.ToUpper,
"TYPE=SUBMIT", CompareMethod.Text)
                   If intPos = 0 Then
                       intPos = InStr(1, strTag.ToUpper,
"TYPE=""SUBMIT""", CompareMethod.Text)
                   End If

                   If intPos = 0 Then
                       intPos = InStr(1, strTag.ToUpper,
"NAME=""", CompareMethod.Text)

                       If intPos = 0 Then
                           intPos = InStr(1, strTag.ToUpper,
"NAME=", CompareMethod.Text)
                           intLength = InStr(intPos + 5, strTag,
" ", CompareMethod.Text) - intPos - 5
                           strName = strTag.Substring(intPos +
4, intLength)
                       Else
                           intLength = InStr(intPos + 6, strTag,
"""", CompareMethod.Text) - intPos - 6
                           strName = strTag.Substring(intPos +
5, intLength)
                       End If

                       If intPos <> 0 Then
                           intPos = InStr(1, strTag.ToUpper,
"VALUE=""", CompareMethod.Text)

                           If intPos = 0 Then
                               strValue = ""
                           Else
                               intLength = InStr(intPos + 7,
strTag, """", CompareMethod.Text) - intPos - 7
                               strValue = strTag.Substring
(intPos + 6, intLength)
                           End If

                           intPos = InStr(1, strTag.ToUpper,
"TYPE=""FILE""", CompareMethod.Text)

                           If intPos = 0 Then
                               intPos = InStr(1, strTag.ToUpper,
"TYPE=FILE", CompareMethod.Text)
                           End If

                           intType = IIf(intPos = 0, intType,
Parameter.File)

                           ReadParameter = New Parameter
                           ReadParameter.Name = strName
                           ReadParameter.Value = strValue
                           ReadParameter.Type = intType
                           ReadResponse.Add(ReadParameter)
                       End If
                   End If

                   intTagPos += 1
               End If
           End While
       Else
           Throw New System.Exception("Form not found.")
       End If
   End Function
End Class

Public Class Parameters
   Inherits System.Collections.CollectionBase

   Public Sub Add(ByVal sFld As Parameter)
       List.Add(sFld)
   End Sub

   Public ReadOnly Property Item(ByVal index As Integer) As
Parameter
       Get
           Return CType(List.Item(index), Parameter)
       End Get
   End Property
End Class

Public Class Parameter
   Public Const Input As Integer = 0
   Public Const File As Integer = 1

   Dim strName As String
   Dim strValue As String
   Dim intType As Integer

   Public Property Name() As String
       Get
           Return strName
       End Get
       Set(ByVal vName As String)
           strName = vName
       End Set
   End Property

   Public Property Value() As String
       Get
           Return strValue
       End Get
       Set(ByVal vName As String)
           strValue = vName
       End Set
   End Property

   Public Property Type() As Integer
       Get
           Return intType
       End Get
       Set(ByVal vType As Integer)
           intType = vType
       End Set
   End Property
End Class

Public Class Conditions
   Private strName As String
   Private strID As String

   Public Sub New()
       strName = ""
       strID = ""
   End Sub

   Public Sub New(ByVal Name As String, ByVal ID As String)
       strName = Name
       strID = ID
   End Sub

   Public Property Name() As String
       Get
           Return strName
       End Get

       Set(ByVal sValue As String)
           strName = sValue
       End Set
   End Property

   Public Property ItemData() As String
       Get
           Return strID
       End Get

       Set(ByVal iValue As String)
           strID = iValue
       End Set
   End Property

   Public Overrides Function ToString() As String
       Return strName
   End Function
End Class
- End Code -----------------------------------------------------

Here's the requeststring generated for the multipart form:

- Start requeststring ------------------------------------------
-----------------------------7d4285126106b0
Content-Disposition: form-data; name="condition"

830
-----------------------------7d4285126106b0
Content-Disposition: form-data; name="notes"

good good
-----------------------------7d4285126106b0
Content-Disposition: form-data; name="image_file"; filename=""
Content-Type: application/octet-stream

-----------------------------7d4285126106b0
Content-Disposition: form-data; name="image_url"

-----------------------------7d4285126106b0
Content-Disposition: form-data; name="version"

729
-----------------------------7d4285126106b0
Content-Disposition: form-data; name="domain_id"

1856
-----------------------------7d4285126106b0
Content-Disposition: form-data; name="meta_id"

1
-----------------------------7d4285126106b0
Content-Disposition: form-data; name="context_name"

w13.1097912464.0000318444
-----------------------------7d4285126106b0
Content-Disposition: form-data; name="x"

20
-----------------------------7d4285126106b0
Content-Disposition: form-data; name="y"

20
-----------------------------7d4285126106b0--
- End requeststring --------------------------------------------

The requeststring looks right to me. I'm wondering if there's a
header I should set that I'm missing.

Thanks for the help.