Informal, off the books, everybody sticks to the story deals are one thing. Sterile lower, guys know each other, never registered. Nothing stops the guy from claiming he made it himself.
On the other hand, you set up a table at a gun show, with three ARs for sale that were built by an unlicensed manufacturer and have no markings, it might be slamsville.
To narrowly read the law, it says the guns cannot be built with the intent to sell. It does not say you can never sell the gun. Fact is, the law is the same whether you did an 80% lower or a 100% lower, because the exise tax is not paid on the 100% lowers either. The only difference is that the 100% lowers have a trackable chain of custody, and the ATF goes easy on them because they would rather have some trackable chain of custody than none. Still, there are lots of holes in the system brought out by GCA '68. Not the least of which is the fact that a trace only goes to the original buyer. If that buyer trades the gun/lower even at another FFL holder, the used gun is not reported by that dealer to a central location or the factory, so even though the second (or third, or fourth or fifth ect) dealer does all the paperwork on the lower/gun, the information does not really go to ATF unless it is in a state that registers long gun sales or the particular dealer goes out of business, fowards his bound book to ATF, and that information gets put into their computerized database.
People have been getting away with the builds on 100% frames and lowers, then selling them, for decades. Millions of 1911s, ARs, FALs, and now AKs have been done up that way. One gunsmith I knew did not even get a peep out of the ATF about it until it reached the point of 50 guns per month. Nowdays, the unofficial number is ten guns per year on reciever builds. You do more than that, and they start asking questions.
This is not limited to an assault weapon issue, but also applies to those gunsmiths that build the $2500 1911s out of parts, and those who make the $10,000 engraved shotguns. No tax was paid on the recievers, but if it left their posession as a complete firearm, and was value added product, then it technically is manufacturing. The thing is that ATF has been easy on it most of the time because they still get what they consider is important from a law enforcement standpoint, which is the somewhat papered chain of custody for the firearm.
What's left is the tax issue, and if you keep the dollar amounts small in relation to what they figure they can snag on forfeiture, they leave you alone. Again, most federal agents dread the idea of having to go into the proverbial trailer park to take dangerous but valuable weapons from people who are not otherwise criminals and probably value those weapons as their sole prized posessions.