Eh i dunno if it would be that hard to implement, just allow you to package up games on your steam account and send them as a a "gift" to another steam user. Hell Valve could also offer a steam wallet service, so instead of fucking around with how to get the other guy to pay you the funds would go from his steam wallet to yours directly, and probably could figure out a way to profit from that (3% of the sale or what not goes to valve) for providing the auction house you can sell your game in. GOG could do this as well, but with their method of downloading might have difficulties keeping the game from the hands of the original owner, though their user base might be less inclined to do so (their general nature is anti-drm, but also anti-pirate). I haven't used any other download services, but surely most of them could also implement game trading services in this way also, and by providing the money transfer service onsite, would even be able to make some money back.