If you have a Gamerush or FYE near you, I'd suggest trading in there, or at least checking them out. Generally, those stores will give you a few more bucks in trade, and with GR's trade 3 get a preowned for $5, you'll be able to pick up another game for only $5 of the credit they give you (they allow you to stack your trad-in credit with this promo at Gamerush).
Just my two cents.
Edit: Oh, and if you don't, expect about the same price at GS, if not lower. If it were me, honestly, I'd try selling them either in these forums or on eBay.