That's not the reason he won't take a credit card. The reason is that you can deny/protest the credit card charge when you discover he lied and cannot get the vehicle he promised in the time he promised. With a debit, he has the money. An "old school" trick of shyster car salesmen is to get some money from you so that you must keep talking to them.

I ws looking to order a Ford Focus and was checking around at dealers. Since I already had a couple do a inventory search of the entire U.S., I knew there were no new Focus Station Wagons with leather seats anywhere in the U.S. The Sales guy went away and after five minutes came back and said he had found three at other dealers and could get it.... but first I would have to leave a $1,000 deposit. I asked to see the printout as I said "I was surprised you found one with leather seats." The idiot came back and handed me a page printout showed the vehicle and had, on the bottom, "press F4 to order." Having had worked at Ford in the past, I recognized the printout as a simple screen shot of the car ordering system, and the salesguy had simply created a new card order for the system, and then printed the screen shot out and presented it as an inventory locate.

I told the guy I knew exactly what he did, laughed in his face, shook my head, and left.

Had I given him a deposit, I would have been forced to continue to talk with the guy which, in his mind, meant he still had a chance to "bring me around."