Is it safe to buy a car that's been in an accident, but driven for YEARS afterward?

Ask the owner if he/she still has a record of the estimate or summary of what was done at the bodyshop. With newer cars, even a small fender bender can damage the outside skin pretty bad and leave the frame intact. The disablement could've just been a blown tire or the person did not feel safe driving it home (or the lights were busted). Ask if the warranty on the body work may still be valid. Should be okay if the same owner that crashed it, kept it afterwards.