Starting today, the capability exists to lock
your own topics (threads) in the Marketplace Forum. In the past, users were encouraged to edit the subject line in their initial post with "SOLD", "FOUND" or other message to indicate that the topic is closed. One of the moderators would then have to see the change and lock the topic. Now, there will be a "Quick-mod tools:" dropdown box at the bottom of the thread with the option to "Lock topic". Simply click the Go button and then confirm the action.
It is still recommended that you edit the original subject line prior to locking the topic. Once the topic has been locked, you will not be able to unlock it. If that is what you need, then contact a mod for assistance.