You may want to simply update your original 'For Sale' listing for each item sold. As new threads enter the page, the 'For Sale' post will be separated in listing order from the 'Sold!' post and it will appear that the item is still for sale. No need to post a new thread to say 'Sold!', just edit the old one.
"All of us have a place in history. Mine is clouds." Richard Brautigan