In the DPRM you have to get a used car dealer’s license if you sell more than four cars a year.I ran into that a lot on Facebook marketplace. You can tell cause they don't know anything about the vehicle. I had that one thread about the guy wanting me to meet him at the gas station, then go to his house after meeting. That means he does this all the time and doesn't want someone coming to the house who ends up flaking and he's stuck waiting
We find many “private” sellers with multiple cars for sale. Usually, they try to get the seller to sign the title but leave the buyer blank. That way they avoid the sales tax - and any prospective liability, because dealers have to provide a basic warranty.