I bought a used Acoustic Bass the other night in the parking lot of a local grocery store. It felt kind of weird in that it felt like making a drug deal. 'Hey you got the goods?' 'Yeah, you got the cash?' 'Yeah I got the cash, show me the goods.' 'Show me the cash, and I'll show you the goods.'
From a security standpoint, it would be best to do any private deals in public. Walmart parking lots are good for this because they have a crazy amount of cameras all recording at the same time. Set a date and time along with a drop dead time. (No show, no deal). Transfer large sums of money electronically. Use Paypal, Venmo, etc. But, not until you have had the opportunity to look at and put your hands on the item you are buying. Know who you are selling to/ buying from. Find/get a picture of the person, if you dont. Get to know them.
Most importantly, when its time to make the deal, if something doesnt look or feel right, just drive away. No money lost or stolen. Keep the meeting short and to the point. Save the small talk and distracting conversations for another day. Its just business. Do business first, be friends later. Bring a friend along to be your wingman. Never go it alone.
These rules can be the same if you are the seller as well.