What if everyone gets to sell their steam games on your site, they set the price and everything. You'll be like an escrow service. You could even automate the process with bots. Where the seller transfers the game onto a bot account and after that the game is listed on your site.
How can he guarantee that the games put up for sale are legit? You won't know if a code is correct until you redeem it and gifts can be bought using carded credit cards so they will be revoked by Steam.