you should prob just put on the mk5 doorcards if its such an issue with people. it doesnt really matter that you prefer the mk6 ones anymore if its for sale.
i wouldnt even humiliate someone by giving them a doorcard changing lesson either. if they need that to try and convince them to buy, they never were going to buy the car anyway and they shouldnt be let near a car.

nice car, good luck with the sale