It's been a while since I looked closely at the legislation, but I seem to recall something about "manufacturing" firearms in it, which may render the finishing process illegal.
When the seller says, in his FAQ page, "there is no ruling on 80% lowers", one should approach with great caution.
The first guy who attempts to defend his purchase and handiwork up to the Supreme Court had better have lots of money to spend on BMWs for lawyers.