I know GameStop buys used games and systems and such, but do they give you a good deal? Has anybody sold to this place, and if so, is it worth my time?

I've bought and sold from this place and another place here in California called Game Crazy.
As far as a good deal? Well it just depends on the game. In both places, the price of a used game varies. If they have a lot of copies of a game, the trade in value is a lot less than if they only have one copy. I.e, you won't get as much for a game if you trade it in.
The thing I like aobut GameStop is if you buy a used game, you have a week to return it for a full refund, no questions asked. Even if you play it and finish it! This is mostly because they don't have the option to play the game in the store. GameCrazy does have that option, but, you can't if the game is rated Mature. So in this way, Gamestop comes out ahead.
If you have a membership, the price on a used game is even better.
I've found prices on used games to be fairly the same, just depends on the game and how popular it is.
My suggestion, call and ask how much a game's trade in value is. Call several stores if you can. There are about three that are relatively close to where I live, about a 30 minute drive.
I went into one store, asked about a resale value on NCAA College Football 2003 for PS2. GOt my price and decided it was worth it to trade it in. Went home and brought the game back. In the course of 30 min, the price dropped down by half because they had gotten several copies in in that 30 min or so. :-(
Went to another location and I actually got $2 dollars more than my original quote. So be careful of that.
Also, they can be real #$%$%$s and charge you a "resurfacing" fee if there is even the tiniest of scratches. Even though the game plays perfectly fine, be careful of that. If you have the stuff at home, resurface the disk yourself.