When you have sold your item, all you need to do is an Advanced Edit on the first post of the thread and add the word "SOLD" to the title, and make one final post to the thread indicating that the item is sold. Doing so will allow your thread to be picked up by the periodic filtering of the Marketplace and moved to the Completed Sales Forum.
Please do not delete the title or the contents of the thread, as that will make it unsearchable, thus defeating the purpose of establishing the Completed Sales Forum. It can also cause issues with iTrader Feedback.
Feel free to PM me with any questions.