Depends on your local laws. In MOST states, a private citizen may lawfully make a "face-to-face" sale ONLY to another resident of his state. If you have cause to believe that the other person may not lawfully possess a firearm (he is 18 and wants your revolver, he just got out of serving a 5 yr stretch at the big house, etc) you may not sell to him. The suggestion to check with a local gun shop is a good one.
What we have here is... failure- to communicate.