It’s not a gray area. The ATF has admitted that you may legally construct a functioning firearm (i.e. the part that is serialized by the manufacturer) as long as you do the work yourself. Since a 80% completed lower is just a chunk of metal as far as the AFT cares, anyone can purchase one without having it recorded in some dealers bound book. Therefore the ATF has no record of it.
Yes fut there a restrictions on sale of said firearm.