It depends on the game.
Games that convinced me to devote an absurd amount of time to them because it felt like they were building up to something good but never got there like Grandia, I will find some fitting way to destroy them in a glorious fashion.
Games that were terrible and had no value for me I sell to gamestop, I don't care how much they'll give me. It is my way of voting with my dollar, I bought this game, to me it was a blight upon the community, and that isn't an acceptable reason to return it to the store to reverse my purchase, so the only way to do that is to make sure they don't get credit for someone's future purchase to balance the sale. example: Madworld
Games that I enjoyed but don't think will have replay value, I'll hold onto until I find someone who won't buy it due to lack of disposable income and I'll give it to them, I'd rather a friend buy the game to add another sale to a good title, but if they aren't going to I'd rather they get to play it. example: Dragon Age
Games that were good and I think will have replay value I'll keep next to my xbox/pc where it will most likely collect dust, and I'll think when bored Hey I should play that, ooh new colbert! example: Mass Effect, Oblivion, any old version of Civilization
Games that were mediocre to bad, but not TERRIBLE, I'll throw in a box figuring I'll get around to finishing it or playing through it again, and not remember it until I unpack that box after a move or two (I frequently never finish unpacking between moves so it could take awhile). example: Beowulf (got it as a gift)