There is a hollywood video by me with a huge game selection that gives fair price for most games. If it's older, don't bother selling it, but if you are selling a game w/in 6 months of release they'll givey ou $15-$30 for it, depending on what they have in stock.
More importantly though, they'll turn around and sell it for... $25-$45, so the markup isn't so frustrating. Also have a 90 day no questions return policy on used games, and if you have their membership card they give you like 10 free CD buffs a year. Sounds stupid but it saved a few old games I thought were dead.