Could someone explain this in more detail, please?

I notice some people add "[Sorry, it's sold]" in front of the thread in the ForSale forum. I want to do this too because people are contacting me for stuff that's long been taken.

If I go to "Go Advanced", there is a "Title" edit box (second box from top after "Reason..."). This only changes the title of the first posting, it does not change the main Title that appears when listed in a forum, and this is what I need to do!