Under federal law, they can be bought and sold like any other gun.

The issue is that you need a federal firearms manufacturing license to be in the business of manufacturing firearms. The occasional sale of homemade firearms you no longer want is legal as long as it's infrequent and you're not manufacturing the weapons to make money.

That I'm not entirely sure on and a cursory search doesn't bring up anything helpful. It probably gets passed down like other effects? But that's just a guess.