Thanks to everyone for continuing to post items in the new MacInsiders Marketplace Forum (1000 textbooks have been listed!). We're still building it and are always open to suggestions to help improve the buy/sell experience.
A feature has now been added that will allow you to mark your item's thread as SOLD. Once you sell an item and want to indicate it is no longer available, you can click the big green button at the top of your item thread to mark it as sold. See below:
Enjoy!