the most frustrating part of it is Steam could easily let you sell it if they wanted. They have the whole gifting games thing already set up, it would be easy for them to let you digitally gift a game you already own to someone else and deactivate it from you.
Steam is beholden to the developers from whom the games come from. They do gifting only for their own games. For the rest, they can only go by the terms set by developers. Steam is merely a distribution system. They don't own the games they process (except for Valve developed games). They can't easily sell them if they wanted. They get a commission on every license they sell. It is in neither Steam or developers best interest to let you gift games, since it defeats the whole point of online distribution. Gifting would basically allow digital freeloading since a group of people could then just keep bouncing a single license to each other. Buts lets assume they allow one instance of gifting per license. Then people would complain that its not fair that they can't gift a game that they received as a gift. Its easier just to say no and leave it at that.

