Rule number one is have a genuine concern for your car and your goal prior to having an encounter. You do not want to have your car stolen (letting someone test drive it and they pull a gun on you say get out) and end up on the 6 o'clock news. To counter act this meet the person near a Police dept in a open parking lot (not a garage.) make sure they have identification and insurance. If they do not and thereis a crash then you may be liable.
Know ahead of time they may want to see what your "1994 Zelchmeister" (make believe name of a car) runs like. You are going to have to figure out if you or the prospect is driving? Are you letting them take it on their own? What possesion of theirs will you retain to assure your return of your car.