People trade their games to Gamestop because they don't want the hassle of selling them online. For instance, maybe they are just a kid, and their parents won't help, or maybe they just don't trust the internet.
If you are going to go to the hassle of putting it online and then shipping it, why not just put it on ebay and make three times what Amazon would give you? I did a quick search of a few games, and Amazon's trade in value is still about a third of what you could get on ebay.
I think Amazon is missing the point.