You'll get the most money selling it on Craigslist, the eBay. Craigslist > eBay because of the eBay fees.
If you want to do trade-in, Amazon will give you the most like you saw but it won't be cash. They will only offer Amazon gift cards in exchange.
Other buybacks are gazelle, technollo, buybackworld, and nextworth.
Or you can bring it to your local gamestop to get cash directly in their store.