If it is real cash, what is the problem?
I would suggest they sign a receipt of purchase, receipt, and acceptance per your conditions of sale spelled out in your ad, if any. With their name, address, etc. on the receipt.
But really, it’s cash, so they have little recourse should they change their mind as long as your ad stipulated conditions of the sale. They would have more recourse using a service like PayPal. Who cares where the money is coming from, that isn’t your problem.
Personally, I don’t like dealing with cash.