I buy used cars in the 15-25K range. I always insist on paying all of it with a CC. I get a lot of flak and SOMETIMES I will let them only get away with charging 15K. I can't let mileage opportunities like that get away. One problem is they won't take AMEX. I guess the fees are too high.
You really have to be firm, not nasty. I've never been refused. YMMV.