I made a reply to a For Sale ad I had placed, in order to have a fresh reply that would bring the ad back to the top of the listing. It's a common practice, with the reply just having the initials "ttt", meaning "to the top". I do it about once a week as opposed to some people doing it daily or more often.
When I do this, I do not see the ad, the thread in my "unread" listing.