There is a plugin that will create a drop down menu that will mark the thread accordingly in the SALES area.
You can add the following and let the members mark their threads accordingly.
If i remember correctly, all sold threads will be set to send to the end.
I would use the following for the drop box:
Want to Trade:
It will allow each member to mark their own thread and makes them responsible for it. Plus keep from old threads popping up with others still asking about the items.
Just a thought, when i ran a forum, it helped a lot and cleaned up the sales area.