Am I responsible for providing a refund if the person wants to take me to court and does not want to handle the situation in any other matter?

I had recently sold a car which is more than 6 years old. Two weeks prior to selling my car I had paid for my car registration where a smog certification by the DMV was not required and a vehicle license plate tag was issued. The buyer, one month after purchasing the car wants a full refund and return the car since the buyer was not able to pass a smog check. I had verbally disclosed any information that I knew about the vehicle and the buyer now claims that I did not mention that the car would have a problem. As far as i was concerned the car was fine and the buyer verbally said that everything with the car from his point of view was good. The buyer also mention that all necessary services for the car would be done on their end. I had also issued all vehicle service information and parts which were replaced during the vehicle services.