This is certainly a strange scenario with no asking price in the ad likely leading to a protracted and highly competitive sales cycle.
I think a good analogy might be that of a favorite fishing hole. In this case the fishing hole is publicly accessible (anyone can fish there), but only a few people like you know it’s a no-miss spot. You don’t mind someone telling a friend about it, but you would likely be disappointed if the location were broadcast widely to the general population.
I’ll often pass a lead to a specific individual if I know it matches their buying/collecting profile. Right or wrong, there’s something about the broadcast nature of a “Hey everyone! Check this out!” that makes me feel as though I’ve had an advantage taken from me. My apologies if this sounds like entitled whining, because in the fishing hole analogy everyone has the right to fish in that spot. And nobody is breaking a rule or law by telling everyone else about it. But that does little to help the disappointment when you lose that unique advantage.