Actually, it is not a crime in most US states. Person-to-person firearm sales are a wonderful example of a seller choosing to assess the intention of the buyer, and the moral ambiguity that this represents. A person can legally sell firearms out of his trunk in a great many places asking for nothing more than proof of age of the buyer.

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].

[1] - http://smartgunlaws.org/private-sales-policy-summary/

> No, because selling guns out of your trunk is a crime more often than not.

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.

Not usually. Private sales of firearms are cash and carry most of the time. Thus the debate over gun show sales.