Here's a sure fire theft detector, really.
Whip out your phone, tell the seller you want to take a picture with them and the item.
Works like a bright light on cockroaches, a thief will scurry for any handy hiding place.
A bit worried about a gas station purchase? Take a picture of seller's license plate. I buy sell old motorcycles, often not running, title jumping is a common thing, title transfer is often more expensive than the asking price on an old barn bike. Yeah I'm sure I've bought an item or three that was stolen. One time in retrospect I'm pretty sure a "kid" was selling off his dad's stuff right out the house while dad was gone or maybe just at work... But folks do get in a cash fix and just need money right now. Small high dollar items on FB or CL and quick cash in hand, faster, better price, less embarassing than a pawn shop? shrug. I've had a few shady characters show up at the ranch, and will admit to having the wife complete some of my sales while I'm off somewhere.
It's not infallible but I do feel I can get a pretty good feel for sketchy buyers. If I'm a bit unsure I'll request a phone call or a phone number to text to. I use my phone number in ads, as a general rule anyone that calls is going to be legit. (and older LOL) More than one buyer/seller has gone poof after a phone request. Good way to get rid of the damned kids with no intent to buy also, a big pain in motorcycle sales. I'm a small town guy and have had few scary trips carrying cash to the "wrong part of town" in a big city. I've also told a few widows they need to stop inviting strangers to come to their rural homes!!!