a) Doesn't belong on the Bazaar.tf Forums
b) Is in the wrong Subforum
c) Breaks the rules of the Bazaar.tf Forums
... it's easier and faster to just report the post with the report feature (yes, it works for the forums as well) than to post "This does not belong here" or "This breaks rule X". The only thing you achieve by replying to the thread is to give the offending user more publicity for whatever they want to post about.
If it's a more serious offense (namely phishing links) it can be helpful if at least the first person who sees it posts a warning to other users, as well as warning staffers directly through steam. In all other cases a report will do just fine.
Your friendly Neighbourhood Forum Supervisor,