Here is an approach I'd suggest:
Ask for gun's serial number.
Get details of the time, place, phone # of seller, name and vehicle description. Leave note of all this info at home.
Invite a friend who can go with you. [If you are w/in 100 miles of Chattanooga, I'm available.]
Put a folding card table, and a bunch of newspapers in your car/van/suv/motorcycle.
Have asking price ready in $20, $10, and $5 bills in envelope. Don't take money out of your wallet.
Have blank general bill of sale forms so you can fill out info for both parties.
Meet w/ the guy at safe place. [Call local Sheriff Office or local PD. See below... they are everywhere now.] Confirm that it is
not posted "No Weapons."
When the guy shows up introduce your friend who should say not much more than a neutral to friendly, "Hi"
Friend steps back and makes a phone call w/in earshot and says "We are on schedule [or ahead of schedule, or a little late]. See you soon."
Friend keeps eye on surroundings, 360 degree awareness, watches for seller's late arriving "buddies." If I'm your friend I take pics of seller and his vehicle tag and email them for evidence.
Friend steps back during transaction. Keeps phone in hand in case of emergency
You are a 590 owner so this 500 isn't going to look foreign to you.
Do visual inspection inspection and operate the action. Set up the card table and have the seller field strip it or change barrels.
When you are done w/ inspection make offer or refusal.
If offer accepted. Fill out bills of sale, keep a copy and give copy to seller. Pay from envelope.
Your gun inspection and transaction is not as important as your safety, security and protection that your friend provides.
Buy friend dinner on way home.
Report back to us at Mossberg owners how well you did. WITH PIX DAMMIT!