With that said, I have absolutely no problem with their practice of buying and selling in the used game market. Many people complain that they purchase low and sell high. I see nothing wrong with this, it's just smart business. They aren't forcing anybody to sell their games, they are simply giving an option. If you don't like it, then don't partake and certainly don't complain.
Of course they are free to do what they want. I don't partake in their trade in, but I don't see why I can't complain a little too.
For me the biggest problem is the convenience gained by trading in at gamestop does not balance the loss in money. As an example, I use Modern Warfare 2 for the ps3. Gamestop trade in value is taken from cheapassgamer, the craigslist value is for the dc region.
Three choices; I can trade the game in at gamestop, at amazon, or sell it on craigslist.
If I trade it in at gamestop, I get $22 for it. While this is the least amount, it also is the easiest (maybe) and quickest (almost certainly) option.
However, if I take a little more effort I can ship it in to Amazon for $33. That's 50% more money for a little extra effort. The gift card is also for amazon; while I haven't done this I assume it is valid with all amazon products, and not just video games (a restriction that I believe applies to their pre-order credits).
If you're willing to deal with craiglist you can fetch around $40 for it. This is assuming that the asking prices are generally agreed upon.
I see almost no reason to pick gamestop over amazon. While gamestop may be slightly more convenient overall, the amazon gift card is superior to gamestop credit. Given that amazon matches gamestop trade in values, price should never be a factor. For some people, shipping to amazon may be easier than driving to the local gamestop.
In the end it's just frustrating; while gamestop is free to do as they will I just cant justify trading games there when I can get a better deal elsewhere. I would reconsider if they sold their used games at more reasonable prices. It is especially annoying to see a game priced new at $20 and used at $18. I suppose any savings is nice, but I still did a doubletake the first time I saw this in a store.