Here is my suggestion as I have done this and have also successfully suggested this to others...if someone Posts about an item for sale and its something you want to bid on the...contact the original Poster and ask if they would take it down and explain to them that you want to try to buy it. I have not come across anyone denying that. A quick message to me and the Thread or Post will be removed if the original Poster requests.
We really don't have that many members here that are still active and even fewer that have the financial means to go after the very very rare items. This is why we have the Trading Zone for members eyes only.
The intentions of members Post are honorable, not intending to hurt anyone. Thara your scenario also works in the opposite way where those that "put in the effort" as you say, has missed something and if it weren't for a Post they would never know. I know you have helped many members through Private Messaging to Listings they might be interested in and that is much appreciated.
Your "Golden Rule" certainly can not be enforced here but a simple solution to that would be to message any Poster and kindly explain the ramifications to the information could be. We are always getting new members and some of those are younger so it would be better to communicate nicely and it would also be a learning experience for those.
I will continue to move any Posting showing Links like these from any of our Public Forums to the Members Only Trading Zone when I see them and then it will be up to the original member asking me to take it down. Sound simple, it does to me