I just got a car and paid $2500 with a card.
I got to the dealer from the Amex website which, to me, meant that they have to take the card. It was REALLY far from where I live so I decided not to start with the "merchant agreement" argument as their price was far far better than any other dealership I checked and I had no intention on not buying the car.

anyway, $2500 out of the required $4000 for the 75K bonus on the AA card are done.