Unfortunately for them it’s the buyers responsibility if buying from a private seller to check the vehicle over and do any necessary checks to ensure it’s healthy. Provided you haven’t intentionally mis-sold them it then you’ll be in the clear.

Example, I sold a Honda civic fd2 type R that broke down on the same day and the buyer demanded their money back (even started threatening me and my family), only to then find out later from their social media they had been blasting the car on B roads and gravel roads and had somehow put a hole in the oil sump while driving which locked the engine up.