A marketplace website

I need someone to help build a website which will allow users to post goods and other users can buy them outright or send them a bid for a lower value than is posted. The buyer will pay my company if they pay the posted price or the seller accepts their bid.? The seller will receive the money less our commission and will agree to ship out the goods within a set time frame. This website will need to be able to search for goods using different characteristics within different regions such as a school, city, state etc.