Lol not sure if serious. Most seeking 80% are lazy tinfoilers, not criminals. It's not a "tiny loop hole" either. The only thing you're not wrong about is turning them away, because milling someone elses lower is not legal.
Regardless of why people want them, there is only one logical law abiding reason to buy an 80% lower and complete it. And that is for the satisfaction of knowing that you built it yourself.
If you are a tinfoil hat guy, just go to a gun show in NH or VT and buy one from a non-licensee with cash and a handshake. Voila, instant ghost gun. Nobody knows you have it. Sure it has a SN, but it doesn't point to you in anyone's A&D book or in any state database. And its perfectly legal. (provided you are a resident of NH or VT)
****Edit - actually, now that I think about it, this could LEGALLY be done in MA. The feds do not require any paperwork on secondary sales between non-licensees. MA does not consider a stripped receiver to be a firearm. So a lower receiver purchased from a non-licensee does not require any paperwork. You would need to find someone to agree to not keep any records, and again, there couldn't be an email or money trail if you want it to be on the down low, but it would be anonymous and perfectly legal. ***
If you want to break the law with plausible deniability, then 80% lowers can be a useful way get yourself into a pre-ban configured AR in states like MA or CT
If you are a MA resident, it would be up to the prosecution to prove the gun was post ban. But as len has pointed out, the deck is stacked against you, and the gain is minimal. A flash hider and telescoping stock.
If you are a CT resident, risk:benefit analysis is much different. New ARs can not be purchased at all. The only ARs that can be sold and purchased or even possessed without a registration certificate are pre-94-ban ARs.
If you are a CT resident and want to possess an AR with some reasonable chance of passing it off as LEGAL pre-ban, then an 80% lower, properly made and properly purchased could be passed off as pre ban. It would need to not have any markings and be of a design that existed prior to 1994. You would need to acquire it with no email or credit card trail.
If this unserialized home built gun has no provenance, then it would be up to the state to prove that it was post ban. With no markings and no email or money trail, they would be at a dead end. (Note - pre 94 guns did not have to be registered per PA13-3).
Years ago, I thought it was wrong to tell people how to circumvent the law. But now with the absurdity of PA13-3, which outlawed the best selling long gun in the country, I've had a change of mind. The last time I checked we still had a First Amendment.
Don