If you've listed a textbook for sale in the MacInsiders Marketplace, and have sold the textbook. You should now see a green button titled "Mark As Sold" in the ad at the top next to the reply button that only appears in your own ads that you have posted. Click that button to mark the thread as "[SOLD]" and close the thread to replies.
Threads that are marked as SOLD will be removed periodically from the forums and deleted automatically by moderators.
If you do not see the green button please let me know by replying to this thread.