New cars have a return policy, that is something that you would find out from dealer. Normally its a week or two. There is a lemon law in most states, that you could find out from the governors office in your state. Used cars also a a return policy, that usually is a week. Depending what is wrong with car you can take them to court. I would find out the lemon law and be able to prove the problems. Good luck. Dont think you can return a used car to a personal individual, just sue for damages.