When someone posts or replies to a post the thread goes back to the top of the page. On a busy topic like the for sale section, posts often find themselves on the next page possibly with people missing it as not everyone will read all the posts. The irony is that its normally bumps that cause the threads to slip from the first page. People will use the word bump or sometimes kick in a reply to get their thread back to the top of the page, sometimes with a littl ehumour thrown in
Think of the for sale section like a greasy pole(if you have witnessed the spectacle).