Should have taken his picture; and then told him he is going to be reported to law enforcement as he was acting suspicious.
See what he would have done then!
Or you could have really had fun with him. Tell him; yes you have a lot of guns you could sell him. He has to meet you at a certain place and time to see them.
Taking their picture/recording them is probably a felony, like it (unbelievably enough) is still the case in many jurisdictions. 8-((