Most people don't know any better.
A minority of them are likely stings. Most of them are ignorant people.
As TexasRifleman says:
The real problem is of course the NFA laws themselves. Since they have no "common sense" attached, and the barrel lengths especially are seemingly random, it's not like one would ever think that a 17 inch shotgun barrel is a felony while an 18 inch shotgun barrel is perfectly fine.
I suspect that many people make the assumption that since you can buy stocks and short barrels etc through the mail then they must be perfectly legal to use as well.
That is what I see as well.
The most common violation is 922r, and you cannot tell many of those from pictures. Even people that know the NFA restrictions violate 922r regularly.
Who would reasonably even think that identical aftermarket parts and identical firearm layouts are legal or illegal based on where the gun or part comes from?
Just buying a 922r compliant gun and putting a new detachable magazine in it can change 3 complaint parts, making it illegal. You could fire 10 magazines at the range and assemble an illegal firearm each time you insert a couple of those magazines because they come from another country. (And many of the best quality magazines or manufacturer magazines do come from another country.)
Someone could purchase a gun barely complaint by magazine, and purchase a few manufacturer magazines to go with it and be violating federal law.
Who would have known the 30 round American made magazine they purchased it with was legal in the firearm, but the 5 round magazine they picked up for hunting made by the manufacturer in a foreign nation makes the gun illegal? Or the handful of magazines they bought to go with the rifle after researching the best quality magazine are illegal to use in that gun because they come from Bulgaria?
Never mind all the other parts.
Few people realize they have to buy the exact same parts they already have but made in a different country to legally install other parts that are legal on unrestricted guns they see for sale or owned all the time.
A legal 922r conversion can be visually identical to an illegal gun in every way.
Another common one is vertical grips on handguns. Take those AR and AK pistols, which often already have rails. Someone purchases an over the counter forward grip sold for AR rifles everywhere, slides it on the rail. They think they just purchased a simple convenient accessory with no legal implications, after all they see them on ARs at the range all the time.
Short barrels are less common. I think a lot more people realize there is restrictions on those in certain firearms. But there is still a percentage who don't realize. Or who install stocks on handguns, especially handguns based on traditional rifle receivers.
The assumption in a good percentage of the general public on many things is that if it is legally sold then it is legal.
If it was illegal they wouldn't be selling it.
They are not restricted parts, and can be purchased most places no questions asked.
They don't realize they become illegal when installed on their firearm, or even at purchase under constructive possession sometimes if they have no firearm that can legally use it.