A newbie who has been posting here for three years?
What they mean is that you (for example) update THIS THREAD every time you have a new auction, by editing the post at the top and/or adding a new reply. At the moment you're starting a new thread every time you have a new ebay auction, and not being very descriptive with the titles.
For example, this thread:
http://chipmusic.org/forums/topic/8800/ gbs-tr626/
.. was made in 2012, but Timbob edits the main post and adds a new reply every time he's got something new to sell. The new reply bumps the thread to the top of the list and the edited first post makes it easy to find where the latest items for sale are (ie. no need to dig through the thread).