I have been wondering how to create a certain website.  The idea is simple.  It will function the same way I would as if I were doing the search myself.  This is what I want my site to do and its purpose (fictional product/service to protect my ideas)
Lets say I want to be the middle man for used pc's but not just used pc's, parts off these used pc's. We will call this site PCpartExchange dot com (or whatever) I want to create a site that would allow users to upload information about the pc parts they have.  There would need to be pictures as well.  There would also be a section for someone to list complete pc's that they could part out. In other words, if some just needs a video card, they don't have to buy the whole pc.  I would also like it to have a search engine that would search the site for the specific part they need, and if the part isn't available there, maybe search different auctions around the web, kind of like how Shoemoney's site Belowabuck does.  I want this site to have a blog and possibly a forum for users to share knowledge/experience and insight in the used pc parts market.  I monetize this site by making money on ads, both placed by user and by adsense or something similar, ebay ad commissions, traffic, affiliate advertising.

What would a site like this cost to design?  Are there sites like that out there that I could just substitute (plug) my product/service in and use it as my own with my domain. 

I have had the idea for this business for some time, but I just can't seem to get it off the ground.  Does anyone out there want to help me? 

(My idea has nothing to do with pc parts)