A hidden threat to anyone buying a used car are missing air bags. A recent editorial in the L.A. Times outlines the problems. Cars previously involved in accidents where air bags deployed are later sold without the air bags being replaced. This problem led to 51 fatalities in 2010.
A word to the wise: If you are buying a used car, investigate to make sure that the air bags are still in the vehicle. Companies, such as CARFAX, are willing to provide a vehicle history, for a small fee so that you can determine if the car you want to buy was previously in a wreck.