Gamestop is notorious for giving back alot less than for what something is worth. At perfect condition they will give 1/3 back.
Example: You go in and buy a 300$ ps3. You walk out and realize that you actually didn't want it and want a Iphone 5. They will give you 100$ if you're lucky. Do not plan on getting alot. You'll probably get more just selling it to friends and such.
Look at what those games currently cost, now halve it. You'll get half of what the price is or even lower. I'd just keep the games if I were you. If you sold all those you'd probably only have enough to get one new game (if that's what you're trying to do). Not worth it in my opinion.
Don't ever sell games at gamestop. You only get a miniscule amount of money back for what you paid for it. You'de best bet would be ebay, look at what everyone else is selling that game for then lower the price by about $2-5