As a student of economics, I am an advocate of the free market in the general sense. While it is obviously far from perfect, in the case of used games the market is sending a clear message: we as gamers are willing to accept a pittance for our trade-ins as a cost of convenience, and we are also willing to pay almost full price for used games. I will not dispute that GameStop engages in predatory business practices, but their survival is still entirely dependent on the tolerances of the consumer base. Money talks, and gamers' money continues to be taken as tacit acceptance of the market conditions GameStop has created.
If we want the used games market to change, WE must be the ones to change it - and we can. If you want your purchase price going to game creators rather than middlemen, buy new; much of the time it is only another 5 or 10 dollars anyway. If you want more than GameStop offers for trade-ins, there are a myriad of marketplaces for you to peddle your wares, including eBay, Amazon Marketplace, and even Craigslist. And if you want to BUY for less than GameStop charges, look at those same marketplaces. GameStop may be a national behemoth, but it is hardly the only game in town.
What we gamers need to remember is this: the customer has power, and no monopoly lasts forever. In the marketplace, we speak with our wallets, and if we speak up, suppliers WILL listen.