Every time someone reply's to a for sale it bumps it on the page. That is why it's frowned upon, also it muddles up the page and things start getting personal on for sale when it should be a buyer and seller conversation only. Wondering why it's so hard to PM someone on an item? It's so easy even a caveman can do it.
I have been asked to offer information and history on cues in the for sale section more than once. I never felt it was a TOS violation. If I ever wanted a listing bumped back up I'd just bring it up top every week. People used to list items so often a thread could get buried pages deep within a few hours.
Seems like starting another thread on the subject has more impact to the server than commenting on an attractive cue in an existing sale thread...
Some people have even found a stolen cues owner through a brief post interaction. Things like that benefit from the transparency of the situation.
Often, people will ask that no one reply or comment and ask for PM only.
Seems like it works for the most part.