Well...I've heard that I can buy a gun at a gun show...but I can't buy a car at a car show...
Because cars are more strictly regulated than guns...
I've heard that...but I haven't looked it up for myself yet...
I've heard that too...but the closest thing I have ever verified is that a dealer was at a gun show in MD (a state restricting in state sales of magazines over 10 rounds) and the dealer sold a 20 round magazine to someone. The dealer didn't know the law and was arrested by ATF on site.
Admittedly, I have only been to 3 gun shows (mostly out of curiosity) and I never saw or heard of the famous "psssst hey buddy" transaction.
ATF is active in auditing dealers and monitoring sales.
A dealer could not explain having a firearm(s) registered to them for sale and then it disappears. They are audited frequently. They fail by not having the gun in their possession or showing the federal form for the sale and legal owner or they are charged with federal crimes and put out of business.
There are lots of laws regulating legal firearms sales and legal owners. But regulatory laws only work for the honest and law abiding. We do a poor job of convicting and punishing criminals in possession of stolen firearms. It is usually the first count thrown out by state's attorneys to facilitate a sweetheart plea deal to avoid court.
I'm sure that there are some legal firearms owners committing crimes with firearms, but not nearly the number of criminals that can't legally own a gun so they steal them or buy them from someone who stole them.
I do wish there was a way to filter the legal firearms owners that have committed terror attacks. I think that San Bernardino and Miami shooters had legally procured guns. But that means they had no criminal records to prevent sale. I also thought that Dylan Roof had a legally procured handgun and that the ATF dropped the ball in clearing the sale to the dealer as Roof had a criminal record...I may be mistaken.
If punishment for possessing a firearm illegally was severe and enforced, it would certainly help.