In this case it seems that the mileage discrepancy arose before the customer owned the car. The customer can therefore claim that they had no knowledge of the discrepancy and would not be liable. The law recognises that an individual can only give assurances or guarantees according to what they know, not about what they don't know. A private individual would not be expected to carry out mileage verification of a car they are buying or selling, whereas a dealer would be. I think in this case OP slipped up and has to take it on the chin.