Worst case meaning you get robbed or assaulted. First of all, check online feedback and talk to the guy on the phone so you have a number and can tell if the guy sounds like a gun nut. No feedback or knows nothing would be a no no. Meeting in daylight in a public place with surveillance cams and whatever video technology you can bring would help. I would like to have a steering wheel lock or somesuch around.
Be discreet in public, bring garbage bags, there are people who will break your nose for possession of a Star Wars blaster.