Prndlus, you said it. You use your intuition.
if I get an odd feeling about the equipment or the seller, I stop looking.
I think this is the heart of the question. It’s about integrity and trust. we each have our ways of evaluating a potential seller/buyer. I just prefer intuition. And then I follow up with dialogue and request for more pictures. If someone were to balk, or have an attitude, I would move on.
you can’t make a good deal with someone who isn’t acting in good faith.