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,

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/ http://smartgunlaws.org/private-sales-policy-summary/