You can always try Gametz, Amazon, Ebay, Local Stores that aren't a chain like gamestop, Estarland and some others.
If you want you can always trade games instead of selling games.
There are plenty of places to sell games online but since they offer different prices and sometimes they pay for your shipping it is best to see the prices for each game and sell them to multiple places or try a site and do a grab bag thing. It fails a lot of times but I've seen it work out sort of.