I sold an old Hot Rod a few years ago for $7k. Guy brought Seventy $100 bills. I brought a counterfeit detector pen and tested every bill. I explained to the guy that I had gotten counterfeit bills in my garage sale before moving and wanted to be sure that didn’t happen again. He had absolutely no problem with me checking.
One other thing. These are weird times, and if I was getting cash that totaled over $10k, I’d advise the bank I was coming, have a bill of sale, contact local law enforcement that you were going to receive the specified amount and advise them that you would be taking it to the bank. Last thing you want is having a Local Yokel pulling you over and confiscating the cash since it’s over $10k.