If you are carrying cash to buy a car from a private seller and you get stopped, it can be seized. It’s hell to get it back, assuming you can. Last car I bought I arranged to have it counted into the seller’s hand at a bank. But given travel distance and days available that isn’t always possible.
That way the transaction can also be documented and notarized for the protection of both parties.
Heck, we paid cash for a car at a dealership, because we had to move quickly, and couldn’t transfer the money to the correct accounts, they didn’t take 3rd-party checks, or allow credit cards. So we just dropped $7000 in cash on the desk.