When I closed my game center, I had probably close to a thousand pre-played XBox 360, PS3, Wii, and PC games (only sold sealed PC games). Instead of losing my shirt trading in at GameStop, I waited until a comic book/anime/video game convention came through town and rented a couple tables (I think two tables were only $50 or $60) and sold them for far greater money than a trade-in would have given.
I didn't sell everything, but I sold a few lots on eBay and then saved the rest for when GameStop runs their tiered trade-in offers so I'd have plenty of $2+ fodder to pad the trade to get the maximum bonus.
Another option might be to use social media to organize a small event (try to get free space in a public building or a low-cost venue like a Legion Hall or something similar to cut down on overhead) for buying/selling/trading games. With more people involved, it'll draw a better crowd and give you a better chance to move your goods. If you're a college student, there might be free spaces on campus that would allow this activity (with permission, of course).