And, when sellers don't know what they have, you aren't getting burned, it would just be a bad deal
I have gotten some great deals when people advertise something like "old computer keyboard" or "old vinyl LP" but you can't expect to be lucky more than 10%-50% of the time, but sometimes you get a gem for a pittance.
On the other hand, I got especially burned once when an idiot woman listed some thing like "lot of 3 clickey keyboards in excellent condition" and put in a blurry photo that you could just almost/barely make out. I bid against a couple of people to something over $100 and won.
I got 3 nasty keyboards that reeked of cigarette smoke: a Model M 71G rubber dome, a Northgate Omnikey that was corroded and dead, and a Chicony 5181 with Monterey blue (the only good board in the bunch).
Her response to my detailed complaint was a bunch of "I am a woman so I am above arguing with you" and "how dare you break down the lot and value each keyboard separately" and the clincher: "what, you expect me to press every button?"
So, when you get lucky, pat yourself on the back and smile with satisfaction. But when you fail, chaulk it up to trial and error.