how do you guys know what to trade? just randomly check games ?
You can go to the trade value section on Gamestop's website, enter any system, and sort by trade value. There are currently two PS4/Xbox One games (which I won't mention specifically) that you can get at Best Buy and flip to Gamestop for a good amount.
It really isn't that hard to do. You just need to be willing to do a little research.