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I would avoid trading anything to GS unless you have no other alternatives; they usually have the poorest returns out all of the avenues that I have utilized when doing the aforementioned trading.
Aside from the obvious Craigslist, Ebay, and Amazon, try seeking out a local Mom and Pop store. I have one nearby and they not only gave me a better trade in value for my games, they allowed for a little bit of negotiation on their offered price.