I'm a GA (peon) at GameStop, and I've been on both sides of the counter for many years. For Game Advisors like myself, we're lucky to get 4 hours a week. I don't work there for the money. Never have, probably never will. I work there because I like talking to people about games and translating geek to common for bewildered parents.
As far as the trade-in "rip-off," It's not always quite the way you think it is. GS gives between 30-50% of what the game sells for used, not new. The acutal amount given is based on supply and demand, just like everything else. While it's not exactly as much as you would get from, say, selling it on craigslist, you also move it out of your residence faster than the turn-around on CL. Also, you are more than welcome to decline trading in a game at any time before the transaction is finalized. For all intents and purposes, GS operates like a specialized pawn shop. Both have to make their profits somehow. That's just the nature of the beast.
I hope I've shed some light on the topic. If you're in my neck of the woods, you can find me at store 534 in Minnetonka, MN. I'm the really tall one with the red hair. I'm impossible to miss.