Originally Posted by UAL_Rulez
Back to the OT, I think the main issue is the credit card merchant fees cutting into the dealer's profit, plus the possibility of a CC buyer "charging back" the transaction for bogus reasons. They don't want to end up with no cash and no car, which theoretically could happen. I'd bet that if the dealer knows you well (i.e., you've bought several cars from the same dealer over the years) they would become more flexible on this.
As far as the merchant fees are concerned, once you are past the "test drive" phase and sitting at a desk, one of the first questions that the salesman will ask is "will you want financing?" If you reply "No, I'll be putting the whole thing on my Mastercard", the merchant fees won't be a problem, they will just boil that into the price you end up paying.
If you say "I'll be paying cash", negotiate a price, and then insist on putting the whole thing on your credit card, you should expect some resistance.