Whether or not Gamestop screws people over is a debatable point. But you can take advantage of Gamestop. Specifically, you can take advantage of three types of human error that its employees tend to make. [Disclaimer: it's been five years since I worked there, but the trade-in system still looks the same to me]
1st: Underlabeling. This happens particularly with accessories. Many accessories, particularly 3rd party ones, have several different trade-in SKUs (Stock-Keeping Units) for one basic type of item. Because the company doesn't spend time training employees on the precise difference between the USD 1.99 GBA light and the USD 5.99 GBA light, when in doubt clerks will go with the low one. Managers don't usually chew the clerks out for giving out too little credit. If you check the used accessories, you can find some good deals.
2nd: Mislabeling. Straight-up mistakes during trade-in are also common; there may be several SKUs for one item, or a game title may appear differently on the game than it does in the system. With a barcode to scan, 98% of the time it will come up correctly. But sometimes it doesn't. Case in point: Gamestop usually doesn't accept Phantasy Star Online Episode I & II (GC) in trade because there is a one-time online activation code (Gamestop's refusal also probably leads to the high eBay prices the game commands, but I digress).
When employee X.X.X. accepted the game for trade on 08.13.07 (information encoded in the price sticker), he or she scanned it. The POS would have made an unhappy noise because PSO isn't a tradeable game, and automatically defaulted to the first game in the "P" section for GC games -- P.N.03. The employee then continued on to complete the transaction as P.N.03, a game that retails for a whopping USD 4.99.
I asked to check the disc for scratches, when what I really wanted to know was whether the game case or the sticker was in error. It was the sticker, and the clerk never noticed the discrepancy. Even if he had, his company gives him no incentive to correct an error that would have resulted in more money for Gamestop because of the next phenomenon.
3rd: Shrink. One excellent way for a manager to get a promotion is by running a store where there is a 100% match between what the computer says comes in and what the computer says goes out. Depending on the store, an audit of the entire inventory may be conducted one or more times per year. The actual inventory is compared to the computer inventory to check for shrinkage (read: theft). In my PSO case, if the manager had discovered the mistake on the shelf and changed the label, the computer would have reported that someone stole P.N.03 because it "disappeared". Managers get no credit for games that "appear". So the incentive is to let mistakes remain mistakes.
Note to budding criminals: you can't just switch labels. That's easy to detect. Note to internet Johnny Cochranes: stores don't have to honor mislabeled products, just advertised prices. False advertising is outlawed because of the old "bait and switch," where stores lie just to get customers in the door and then sell them something else.
Oh, and do you know what they do with all the extra inventory that the computer says they're not supposed to have? It gets thrown away. And no, they won't let you keep anything. My district manager smashed every surplus game, controller, and console with a hammer. I almost wept for all those shattered DC games. Even now it brings a tear to my eye...