A tough question. I don't think there is an easy answer.
On one hand you have sellers asking many thousands of dollars for machines but provide vague responses to legitimate inquires, take shitty pictures, and expect people to travel from hours away on that info hoping for the best. Not a big deal if you are local but nobody wants to waste their time driving a long distance to find an over priced, misrepresented turd.
Then you have buyers who want an exceptional deal on a game where a knowledgeable/friendly seller is asking a legitimate fair market price (or better). Asking tons of questions and asking for a million pictures just window shopping.
I think the easiest way to figure out the buyer/seller's legitimacy is to pick up the phone and talk to them. It's quicker and easier to weed out the flakes that way.