i think it's nice idea but so difficult to do that.
I search a website that is inverse of ebay.
In other words, shopping websites generally are used to sell something. I want to design to buy something. I mean someone can put up a notice to buy something. For example I want to buy a car price between $10000-12000 and it should have these features....
Of course these features provide categorization in this website.
If you know a website like this I will be so happy.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic