Not sure how the guy was making money on the mess he was doing as I understand it. It looks like he was purchasing parts and assembling them to sell in quantity. Parts aren't cheap and it doesn't make a lot of sense to me. I guess he had a thing going somehow but he should have known that he was in for trouble if the wrong people found out about it.
If the agent told him he was buying to resell to a friend that is a straw man purchase and highly illegal. The whole thing stinks. The poor guy must have been mentally challenged and of course that's not an excuse.
The laws are such that it's just about impossible to not break some law in just about anything we do. That's where you have prosecutorial discretion and the prosecutor can choose his victim at will. Unless you happen to be a Clinton or and Obama you are at all times in jeopardy of offending some obscure law. Where firearms are concerned they are hyper sensitive and the statutes overlap enough that peril is ever present. You don't have to go looking for it like this guy did.
ATF comes into a pawn shop and does their check. The agent goes to the owner and says this 4473 is in violation. Owner, Why? Because it's in error, the state cannot be abbreviated. Owner, well the post office allows it and the mail still gets here. Agent, Ok I'll shut the doors to this place and place you under arrest until you figure it out. Owner, Oh no That's OK, sorry I didn't understand. Agent just make sure I don't see this problem again. Owner, Sure,, you bet I'll make sure of that. Right or wrong...Well let's just say they are never wrong.
The "solvent trap" thing was another thing that lots of people got busted for. They didn't actually make a suppressor they just made parts that were easy to turn into a suppressor. They had to know it had the potential to turn nasty. And it did.
Don't mess with the tax man and don't mess with the ATF. In the case of firearms they are one and the same.