I just did this too. Went to Walmart first and got SpiderMan 2 and Need for Speed Rivals for PS4 (was on the Reddit list of games with high trade value). Then went to the GameStop with those and my own games (BOTW, MK8D, some 3DS and Wii games, some Xbox One), and was able to get $380. Then used that to buy 4 3DS games (2 were free) and traded them in to another GS and now have like $460 in credit.
Both GS employees had no idea what I was trying to do, were just impressed that the trade-in credit was really high. I also tried to use the credit to buy Amazon gift cards, and he said lets try it, but it wouldn't go thru for whatever reason, so just got GS gift cards. Gonna go to another GS later to use those to buy Amazon cards.