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.
I clearly didn't think this idea trough. I didn't account for people being fraudulent. However there are services that list different people selling keys, I cannot recall the name of that site. My guess is that they have a reputation system in place, where people are awarded reputation for being trustworthy. Steambitshop could at least use that feature. That would mean a complete rework of this site. Even if this is done, there is no guarantee that it would be as successful as the other service that I mentioned.