And I've been pleasantly surprised at times.
This older fellow advertises a Pentax MZ-50 camera with a cheap kit zoom but with 3 absolutely mint prime lenses (FA 24/2, FA 35/2, and FA 50/1.7) in boxes, unused since purchased, gifts to him from a daughter who overestimated his passion for photography.
He asks $150 for the lot.
I phone him up. He says he's overwhelmed with responses (all by email; I was the only one to phone him) and some young college kid is coming over right away. He cannot understand the haste. The seller thinks the value is in the camera.
I explain that the prime lenses are worth a lot of money. I email him references from PentaxForums and eBay and leave him with the idea I will pay a fair price for what I said sounded like superb equipment in perfect shape. We talk about what we like to photograph; turns out it is family, me my young children, he his young grandchildren. Common interest and a pleasant conversation. Sadly, his eyes cannot deal with a viewfinder anymore.
He phones me back in the evening and tells me my honesty was refreshing. $250 for it all and it's mine. The money is not important and he was upset with the college kids who tried to push a sale without any ethics. He wants his daughter's gifts to go to a good home.
My score. I still have 2 of the lenses and the camera as a backup film unit. I emailed him photos of my kids shot with the lenses. His response was very gratifying.