I used to sell my games to GameStop, but they rip you off. Unless you beat it in a day and quickly re-sell it to them, it depreciates.....fast. Don't even get me started about Sports titles ($4.00) trades.....are you serious.
Oh well. I say, keep them and tout your immense library. Or utilize a resell method such as Craigslist to get a better sale to Gamers who do care.