It's far more akin to a guy who sells guns out of his trunk in a dark alley. Chances are, it's being used for crime, even if there are legitimate uses for it.
While I don't necessarily believe that would lead to acts of malice such as confiscation raids, it has already led to bonehead moves such as publishing over the public network the names and addresses of all registered gun owners in an area.
Also, people buy illegal moonshine when legit liquor stores are available. People buy cigarettes sold in the wrong state or from a tribal reservation. Buying a gun from an alley is a possible choice for someone who would otherwise prefer to bypass taxes, paperwork, and bureaucracy--illegal, of course, but not necessarily a prelude to a different crime.
So is operating an unlicensed money transmission business -- in the US, this is likely to be both a state and federal crime -- or a licensed money transmission business without adhering to appropriate anti-money-laundering controls.
> This is a poor analogy.
Seems to be a spot-on analogy.
This method of sale represents ~40% of all gun sales in the US. It would be safe to assume that the majority of guns purchased with the intention to commit crime are purchased person-to-person. There's a good breakdown of nationwide person-to-person regulations here[1].