I have live in the Philly area, and I have seen some dealers post that they do not accept CC's for automobile purchases, or in other cases they limit the amount (say up to 5K). They do this for many reasons including the fee they pay mastercard, as well as the issues they run into if the charge is disputed.
Remember they can always tell you sorry we will not sell you the car. And I think they would do this even in this economy.