Unfortunately, you're about 2-3 months or so too late to get any halfway decent values for your games from DealTree/Best Buy, since most of the values hit rock bottom recently.
I don't even think they have a video game trade-in button on their trades page anymore, since they're supposedly revamping the program now.
But there's also Toys R' Us and they take games in-store instead of having to ship them in.
Although according to the one website Best Buy may be starting to take trade-ins for gift cards in store as of August, which would be much better than having to mail stuff in and wait.