I bought and sold a several components on usaudiomart. It's about communication, you pretty much have to figure out if the person on the other end is trustworthy. Ask some questions that aren't realistic but the responses will be telling:
"I will be in the area, can I listen to it?"
If the response is: "it's in the shed, I just had surgery, I can't get up, well.... why would his/her priority be to sell this thing."
As a seller, I can be picky too. I don't want tire kickers in my home, and will generally ignore such buyers, unless I know you. If you don't trust a seller's feedback history, why even bother?
Even if you try this on a 0-feedback seller that has intent to defraud, they might just call your bluff. They can cut contact any time and what do they have to lose?