The "For Sale" prank will work best if the victim doesn't have easy or quick access to his car for a period of time, hopefully at least a few days. Maybe wait until he goes on vacation, if you really want to freak the person out. Pick somebody with a car that's visibly parked outside a house or in a driveway, put a For Sale sign with a cheap price on the vehicle, and put that person's cell number. People will be calling him for the steal that they just saw, and he'll have absolutely no clue what's happening.