To be fair, in Romania I was refused too, ~5years ago. All cards have a percentage fee - when you spend tens of thousands on something, even a small percentage adds up. Thus the car dealers require a bank transfer, which is generally free of charge for both parties.
It looks like the change in 2015/2016 to reduce card fees, while enabling us to buy a single bottle of milk by card with a 0.2% fee for the merchant, has made things worse for people selling cars.
Instead of a small, fixed fee (e.g. £0.10) it's now also 0.2%.
Typically the payment happens post-haggling... and by that point 1% of the price might be their entire margin on the sale. I mean, if you pay full list price, maybe they won't mind you paying with card :D