Rule 1) No bumping of threads (not even in disguise!).
There is no need to keep your thread at the top of the page. When people go looking for used gear, they will browse more than just the latest threads. If after a number of days you have had no responses, it just means people are not interested in the item at this point in time. However, if something changes in the details of what you are selling, such as for instance a price drop, or if you want to add pictures of the items for sale, it is perfectly alright to add to the thread.