But you can say that about any kind of car, not just Teslas. You could have a guy fiddle with critical system X of a car (maybe suspension or steering) that if done incorrectly could endanger the lives of several people. How do you ensure that a used car is safe to drive? This is a solved problem: you bring the car to a professional mechanic and have him inspect the car, or inspect it yourself if you have the right knowledge. For a Tesla it would be no different; for the case of autopilot a mechanic might inspect all of the sensors (I'm sure Tesla has a utility program for doing this) and then flash the stock firmware onto the car.