There is a chance of having to do additional Fed reporting with a cash transaction of that size. CASH, not a check. Dealers don't want to be bothered. YOU may get a call asking how you accumulated that pile of cash (through legal means). It's not a big deal, but it's a deal.
On the contrary, essentially all of my person-to-person vehicle transactions over the years, buy and selling, have been for cash.
Most entertaining was when a seriously black Black fellow from some shit-hole country in Africa bought my Suzuki Grand Vitara. He paid me in hundred dollar bills (50 of them). His "deal" was buying Suzuki GV's and shipping them through Baltimore to his native country, where they had the reputation of lasting forever, handling any terrain, and they had a good parts availability for those vehicles.