Yes, but no one stays at a yard sale for more than 2 or 3 minutes. By the time the tow truck would arrive they would be long gone. This is at its heart the reason why this is an overzealous non-problem.
Of course, most people don't buy. At least for me, I probably only buy something at ever 4th or 5th yard sale I go to. It's easy to walk in, scan the tables, realize there's nothing of interest to me, and leave.