the optics of large cash transactions are still dicey.
even if you are doing nothing wrong, make sure your not going to be hassled
Do people actually show up with a briefcase of $100 bills like in movies?
If it’s a “nothing fancy” car, maybe a 30k accord or less, and he has a good income, it’s not going to raise any flags with either state or fed.
Bottom line - pay your taxes and you’ll be fine.
I can’t emphasize enough how good a feeling it is to have the title to your own car.
--edit (a word)--