I can't tell you what a crime it is that game stores like GameStop buy a used game for peanuts and then turn around and sell it for just a hair under the new full price. STOP SELLING YOUR GAMES! It's the only way
Sell your games through places like Craigslist. Then you can sell it for more and the other guy can buy it cheaper.
Google 'Game Trading'. There are websites out there that will let you and other users trade games through the mail.