Upload & Sell: Off
Depending on how quickly new threads are being added, a post can go from appearing on the first page of a forum pretty quickly down to being on page 2 or 3. In the Buy and Sell forum, people interested in merchandise are anxious to have it stay in view of as many people as possible----the first page.
By posting any comment, even just the word bump, the thread is moved back up to the top of the cue. By using the word bump, the poster is acknowledging that there is nothing new to say. He or she just seeks to restore the thread to the wider audience.