If you trust the guy just give him the gun and he will give you the cash/crack etc... If it is a stranger I would get a notarized bill of sale. I think they were trying to find a standardized bill of sale around here last week not sure if they did.
I sold an AK to a guy I didn't know . My son knew the family so I was cool with it . My wife typed up a bill of sale listing the model and serial number of the gun and the price . Made two copies . Each of us signed both copies . Both parties are covered .