Gun buy backs are dumb. They usually end up colleting guns that barley work and since guns are still being made its just a suck up PR at best.
Around here the gun shops set up shop right next to the cop table. Anything that would have been worth anything gets bought by the gun shops. The guy that runs the range I shoot at was saying he can get some crazy deals because people have no clue, offer then $225 vs the $200 the cops offer (or whatever,) clean them up and then resell them. He even had some of the WWII era rifles that he bought for $225 - $250 on display. No idea what those were worth but I would imagine collectors would pay decent cash for one in working order with the original slings, magazines etc.