Any business has the right to refuse service to anyone they feel could be suspect.
IMO gun dealers have a responsibility to use good judgment when selling guns.. There are plenty of whack jobs eligible to purchase guns.. Doesn't mean they need too..
Ppl at my old job used to say they wouldn't be surprised if this one guy came in and started killing ppl.. My response was always 'I wouldn't be as mad at him as the dumb ass at the gun shop who sold him the damn gun'
Common sense.. USE IT