Thanks for the link Robin, helpful to newcomers. So here's the question, is "what are you replacing it with" the same as "why are you selling"? I am inclined to answer no, they are not the same question at all. The first opens the can of worms that is continued in your quote of the rules and we can all see the possibly improper connotations it might lead to. Perhaps "why are you selling" is open ended enough to avoid the stench, what do AC members think?
Both questions are used as examples of what should be asked in the form of PM's and not appropriate posts to confront a seller in his for sale thread AFAIC.
BUT ultimately it's up to the seller to notify the moderator if he/she wants them removed. Chances are if the seller doesn't complain, then the posts will stay regardless of the posting guidelines.
In my eyes tho, it's just showing a fellow member some consideration to ask certain questions in a PM.
Cheers,
Robin