You can only trade games that were gifted to you and they haven't been open yet. So that means you would be buying a 60$ game and trade for a 20$ game. And once you have open your gifted game you can no longer trade it. Valve is not stupid like you guys think.
Also, Fallout 3 and New Vegas are the shit. So much fun. Especialy blowing a the legendary deathclaw's head off in VATS.