If you click on thread tools in the toolbar at the top of your thread, you can now select "mark thread as sold". This will change the prefix to [SOLD], as you can see... as well as auto-close the thread.
Try to get in the habit of doing it. I'm sure David, will be doing his regular house cleaning, but this should help every one out, and help keep things a bit more tidy in here until i can get around to re-vamping the marketplace section a bit.