Use a game trading site. I've tried a couple, and the best by far IMO, is Goozex.com
You send out games you don't want and accumulate points depending on their MARKET value (not rape-me-in-the-ass trade in value @ gamestop), and use those earned points to request games that you want, (which are also worth market value). The currency isn't $$, it's goozex points.
You pay shipping (usually $2/game) when you SEND games to other people, and it costs $1 when you REQUEST a game you want. Great way to get rid of stuff you haven't played in years, and you save a ton of cash to boot. Plus no middle man bending you over.