In any event when you sell a car and inform the DVLA of this, now the rules have changed and the DVLA cancels your old road tax you paid on the car and sends you the remaining due for the remainder of your year as a refund by cheque to you at your V5 registered address. The new owner then has to tax it afresh. the "old" way of selling cars with x months left on the tax has long since gone.
If you never received any cheque then maybe you never taxed it in the first place......