Clocking - This illegal practice involves reducing a car's stated mileage to increase the selling price. If a car's mileage cannot be verified, ask to see its service records, old MoT certificates and all other documentation.
Also check the steering wheel, pedal rubbers, gearlever and driver's seat for signs of excessive wear for the car's mileage. If the two appear to be at odds, walk away and look for another car.