Every private sale I have ever witnessed or in some way been involved in, we did a quick circuit court lookup to see if anyone had a criminal history. It's public information.

Most of us don't want our hands, names, or fingerprints anywhere near a gun that might be used for a crime, so we don't sell to shady ass people, and I've never met a gun store sales person or owner who wont straight up deny someone just for saying the wrong thing or even looking suspicious.