Hello russp
I was thinking this myself as anyone can tax a car online and you dont have to be the registered keeper to do so and if the previous owner has declared the car sold to the DVLA and got thier refund then what happens to the tax you have applied to the car whilst in trade?
as yes you cant get the refund yourself as your not the registered keeper so i can only presume the remaining amount of the tax is left to the DVLA
to which you as the seller can only put the cost of the tax you have previously paid for on the asking price im guessing to help refund your money that way