Burt had a buyer lined up for he RPG. He tells Michael that in the chopper when they reinforce the tower. My speculation is that the buyer was the family.

I do not think an RPG is legal. It would not be on display. Nor was it in the secret vault. It was in a smaller safe ready for sale. I suspect there are lots of sporting goods shops that sell guns in LA, but Latch and Scratch cross the width of LA while it is tearing itself apart to target Burt's shop.