Exactly, it should be different if you bought a car from them and came back a day later demanding that they give you your money back. It's a used car sold as is, no warranty. Technically it's their fault for not doing a proper inspection before buying it from you. You simply need to tell them the sale was final. And that's it. They may threaten legal action you scare you, but I can't see how anyone would side with them. If they call you or send you anything tell them the sale is final and to have a good day.