The short answer is yes. You can buy cars, trucks and tractors for cash. The more expensive the car, the easier it often is. Luxury cars in particular are routinely bought and sold for cash.
Literal cash, as in actual paper pieces of money, is not a common medium to do so.
Some dealers might be willing to do this for you, but most will not. They will direct you to your local bank to deposit the money and get a cashiers check instead. They do not want the liability of it all. Perhaps better chances at the Ferrari dealer you've bought 14 cars from over the past 30 years I suppose?
I asked my (luxury) dealer if I could pay cash the last time I bought a car and they basically said “hell no, we haven’t done that in over a decade”. The risk of being caught up in some drug money investigation or whatnot is too great.
Coincidentally showing up to your bank with a duffel bag worth of cash to deposit is a great way to both get your accounts closed, as well as be added to a blacklist so it will be very difficult to open an account anywhere else.
Cash? As in hundred dollar bills?
But it doesn't take too many cash purchases that are not inline with your tax returns before somebody is going to start snooping around.
edit: although sometimes, a lot longer than one might expect.
Citation needed? Where did you hear that this is a routine occurrence? That seems risky for everybody involved, and it requires a report to the government from the seller.
A dealership may take issue with it but a private party accepts no liability by taking cash.
Secondly, while it's possible to construct a private party transaction in the US where this is fine, if the person spending the duffel bag acquired the money illegally, you the seller are liable if you should have known. "Willful blindness" when accepting illegal proceeds makes you liable too. See 18 U.S.C. § 1957
Maybe I misspoke by saying you would be liable for laundering specifically, but certainly accepting that money is a crime if you have any reason to think it was ill-gotten. And that's a huge risk that no one wants to take on.