Pick a mutual location that makes both people feel comfortable. Maybe a local gun club. I am lucky, there is a friendly gun store about 1 1/2 miles from my gun club.
The interested party(s) should be able to examine and possibly fire the firearm(s).
If it is long guns you can exchange receipts (or not) and exchange money and/or trade and you are done. On the receipt have the gun serial number, model, caliber, price, date, and at least the name of buyer and seller if not their complete contact info., and a SIGNATURE.
For pistols I head to the nearest gun store so that in PA the FFL can transfer registration. Arrangements about who pays for the transfer should be made in advance.
I have done this many times and have always offered a hand-shake, 2 week, if you have a problem we'll work it out together promise. There has never been a problem. (The first step usually takes care of the last)
Have fun - good luck!