My advice in selling hand guns to people you do not know is to make sure they are Maine residents and make sure they are over 21. I would put into any ad you place that the buyer must provide a Maine DL. I would also copy down the info from the DL into a bill of sale and have you and the buyer sign it with the serial number and description of the handgun.
There have been cases of people from away coming up to Maine and buying guns from Uncle Henry's to sell illegally out of state. This will cover your ass and deter the wrong kind of buyer.