Hmmm, my mind is racing with the idea of a non-profit that is set up to “buyback” guns. Then I could donate to the non-profit a chunk of cash, then rent a van, put up a big gun buy back banner near one of these places and buy these guns up. Say offer $200 to $250 for assault rifles then turn around and sell them for a profit.
If you are going to sell them for a profit, you could be charged with being a dealer and requiring an FFL.
It is best to use the guns for a year to see if you like them. If you decide that you want something different after that period, then you are not operating as a dealer.
Similarily, if you are merely upgrading your collection, it is not generally considered as being “in the business”.
It is rare for AR or AK type rifles to be turned in. You might get an SKS or three, almost as likely to pick up an M1 carbine or Garand.
Someone could easily end up with a Colt Python or a WW II Luger. At the Phoenix event, one fellow picked up a Colt Delta Elite in 10mm for less than a hundred bucks.
It is quite likely that someone will turn in bricks of .22 ammo for nothing, just to get rid of it!