I know several people that are selling their firearms and buying exactly the same firearm off book via legal private sales.
Example: I have a Glock 17 9mm. I have a pile of mags, holsters, ammo and other accessories. I am now looking for exactly the same gun to purchase in a private sale and then sell mine to someone else in a FFL sale.
Cash remains even and I am now off book.
Lather, rinse and repeat.
Now I can't/won't do that with any of my class III items. And I won't with modest items like shotguns. It's easy to just get more 10/22s and Mossberg 500s.
It's also a good idea to have a quantity of guns to turn in when it comes to that.
It could take some time, but if you don't have at least a basic quantity of guns
off books this will allow you to do it without it costing a ton of extra money,
--Rule Number Two: Be Prepared