Private transactions of used guns are about 13% of gun "sales"; you could add about 3% swaps and trades of used guns among private citizens for total of about 16% of all gun acquisitions among the general public according to the NSPOF survey.
Also according to the NSPOF, among law-abiding citizens, 4% of all gun acquisitions are at gun shows plus flea markets. Most of the gun show tranactions I have witnessed have been at licensed dealer tables with 4473 and NICS checks. A number of private sales/trades do occur at local gun shows but they are not a majority of transactions.
According to a Bureau of Justice Statistics survey of state convicts, 1.7% of all criminal gun acquisitions are at guns shows plus flea markets.
More important criminal sources are fences, burglars, drug dealers, smugglers and other dealers in contraband. Criminals do buy used guns "off the streets" a lot, often from fellow criminals, or from familiy or friends of the criminals, and from total strangers who don't care who they sell to.
But the "gunshow loophole" is one of the least important criminal sources of guns.
The "closing the gunshow loophole" is a lot of hype, a focus of effort in the wrong direction, and ultimately a policy with unintended consequences.
I am convinced our "leaders" are idiots or liars or both.