Large parts of the Consumer Rights Act don't apply when a car is sold privately. For example, the car does not have to be in a satisfactory quality or fit for purpose. However, you must have accurately described the car when you sold it, such as the number of previous owners it had. You must not misrepresent it, for example not disclosing that it has been involved in an accident or providing a false service history. If none of this applies to you, legally you don't have to give the buyer a refund, the onus is on the buyer to check the car is of satisfactory quality, this is why some buyers opt to have potential purchases inspected before they buy.