The perfect description lies somewhere between brief and exhaustive. Of course, simply saying that the car ‘works fine’ and listing your price is not going to be enough, but you should also avoid producing essay-length descriptions. Discuss the reason you are selling it, its features, recent repairs, and defects (if any).
It is crucial to be straightforward when listing your car’s condition, but, again, no need to go overboard discussing its flaws. Sure, you should be honest, but you should not list down every tiny thing wrong with the car – buyers understand that they are looking at a used car, and everything cannot be perfect.