My first question was, what is an unlicensed dealer? I came up with the same definition you did. Second question, how do they know an unlicensed dealer sold weapons? If I sell or give one to another, I am certainly not going to tell anyone.
Some people, when selling a personally owned firearm to a stranger, will use an NICS check to ensure that they are not selling to a felon. The Virginia State Police often have a table at gun shows to facilitate this. I would not sell to a stranger.
If selling to a friend or relative ... I already know. And as for the po-po ... it’s none of their bloody business.