You completely misread (or inferred a meaning based on your own misconceptions) to the OP. The gun trade IS a multi billion dollar business for CRIMINALS. Nowhere in that statement is it even implied that gun manufacturers are allowed to sell firearms to criminals (they can't).
Every state and federal law all prohibit the sale of firearms to convicted felons. Every state and federal law requires gun dealers (manufacturers provide to wholesalers who then proved legal FFL dealers) perform a background check prior to the transfer of a firearm.
Criminal gun trade is just that, ILLEGAL. Criminals gain the weapons for that trade through theft or via straw purchases (paying someone who isn't a convict to purchase a firearm for you) which is...........
wait for it........
ILLEGAL.
It is IMPOSSIBLE for a felon to LEGALLY obtain or possess a firearm.
The issue that many people (myself included) have with the never ending attempts to make it more difficult for law abiding citizens to obtain firearms is that the reasoning always given by politicians is that they are trying to "keep guns out of the hands of criminals" when the laws we already have do that. What needs to be done is to more vigorously enforce the laws that are already on the books.