The onus is upon you as the current registered keeper to inform the DVLA that your car has been sold. If you do not, you will continue to be responsible for taxing the vehicle and any penalties for its non-payment. You�ll also receive mail relating to any offences committed in the vehicle. Simply send DVLA your vehicle�s registration document/certificate with the new owner�s details and they�ll send you a letter of receipt confirming that you are no longer liable for taxing it