I don't know if this has been suggested before, so here goes:

A button in the Topic Tracker section that can move your topic from the Trade Forum to the trade archives.
IMO this would have the following advantages:
1. Clear up Traffic from the forums: once the thread has served it's purpose, others won't have to sift through unnessary topics.
2. Clear up confusion whether or not the ISO, FT, or service is still active. Once the trade is done, it gets deleted and nobody has to make extra posts asking if it's sold.T he original poster won't have to worry about pm's asking about the status of the items in question.
3. Put an end to Spamming threads. If you post something and it turns into a spam war between others, just shut it down without having to wait for a mod to notice it.
4. Clear up Traffic from our own topic trackers: If an item you're watching has been removed by the OP, you can delete it sooner and move on. It won't be sitting in your topic tracker while you wait for updates anymore.
5. Lighten the work load for moderators: Users will be shutting down their own threads, decreasing the overall traffic and making the problem topics stand out more for mods to find.
Let me know what you think!