the problem really is for forums that is more on buying and selling and other forums that needs with some privacy.
as a seller, you post there some info of your item.
then lots of users will post,
"Please PM me details"
there are details you cannot divulge it publicly, it must be made thru Private Conversation.
with 10 or more users doing that,
you need to click their username, then click Start Conversation
why not each post have "PC" link so that, you will just click that link.
also quoting even just the URL will help to identify which topic your are asking,
that avoids confusion if you have lots of items you are selling.