If this is in fact a legitimate question and not just an attempt to get
traffic on your site (which doesn't work by the way)...

May I recommend:

1) Google.

2) Make it look like a legitimate question.

3) Post your question in a forum more related to your question.

This not something you can get an answer for here, as its just too complex
for anyone to type a response.  If you are tryting to design a site just
like that, and asking those type of questions below then you probably need
to learn some of the basics of web site design.  Buy a book on asp.net web
development for beginners.

Alibaba and sites like that are nothing but categorised contact lists with
search and a facility for account management.  The actual site design is
really fairly simplistic, and you could probably hire someone to build you
this very cheaply, or if you are a competent web developer build this type
of site quite easily.

Regards

John Timney (MVP)
http://www.johntimney.com
http://www.johntimney.com/blog

A web development tool.  A specification.  The ability to write websites
would be a big plus too.

At the beginning.

Depends on what you develop it with.

Wear a condom whilst developing the site.

www.amazon.com
www.rentacoder.com

or

www.ineedhits.com

Pete