I don't see the harm in being able to reply to FS posts. Why all the concern about replies? I can understand the prohibition on frequent bumps and "will you take $x" replies, but if someone has a question about the item, someone else is probably wondering the same thing, so why not allow it? And why not allow a weekly bump vs. having to resubmit a new topic? Another legitimate use is when a potential buyer is unsure if his emails are getting through. (Which is why it's rude to disregard emails, but that's another topic.)
And I'd like to allow "are you serious about this price?" posts, but that's just a fantasy.
