Funniest one was when I bought Ratchet & Clank for $10 at Gamestop. Then they had this crazy promotion a week later where they were paying $20 for the game so I sold it back to them. Then PSN was running the game for $6 at the time, so I basically bought the game for -$4.
Also in that promotion I sold them Bloodborne for $20, The Last of Us Remastered for $20, and Uncharted Nathan Drake Collection for $20. Then bought back my copy of Bloodborne for $20.55 after tax and my copy of The Last of Us Remastered for $20.55 after tax, then used their Buy 2, Get 1 Free deal on used games to get back my copy of Uncharted Nathan Drake Collection free. So basically $18.90 in store credit for letting them hold on to three of my games for about a minute and a half.
Gamestop runs the stupidest promotions sometimes. There were people going and buying games at Best Buy and Walmart for like $20 and trading them in for $60 trade credit in that promotion. People were using it to get $450 Switch + Zelda + Pro Controller bundles for like $200 out of pocket, and this was when Switch was impossible to find in stores (hence the need to buy bundles). So sometimes you can exploit the out of that re ed company.