If a "rule" is worth having, it should be published for all to see. Unwritten rules are, by their very nature, ambiguous.
If I see an item for sale or at auction that might be of interest to a particular member, I PM them with the information, just did it earlier today. A week or so ago, I PM'd Calhoun with info on a 1920 in 250-3000 that I ran across, was that against "the rules" too?
Nope, pm'ing info on a gun is perfectly legit. I've lost count of how many guns I've referred to members here.
And the guidelines are in the "Misc. Good Info" thread in this forum, so they are "published".