As you point out, John, just because it's
listed at a certain price doesn't necessarily mean it
sells for that price. Same with Manfred's $CAD900 lens. It would be interesting to follow that particular item and see what the seller actually got for it. But generally, I agree with your point- and put it down to human foibles. We always think something we own is worth more than it is to someone else. Take one of your treasured antiques to a dealer once and see what he says the market price is. As I have been a number of times, it's shocking. And half the time, I end up keeping the item because I'm not willing to let it go for a price that represents its value on the market.
As to things new or almost new, I tried a grip on my 5d4 and absolutely didn't like it, so took it off and put the thing up for sale. Same thing; new, in original packaging, original manuals and warranty card, but no takers at the price I wanted. So, I still have it!
Zen