I think it would be very complicated to set up.
Since ebay were recently sued for allowing counterfiet goods to go through their system, I think a lot of legal advice would be needed.
Unless you're planning on using an outside escrow, you will also need advice on money.
Data protection too, seeing as you will be holding peoples details.
I think you're getting in way too deep here, but then the only website I ever made was a stupid coolpage site, lol