To clean up the classifieds sub-forum, an easy idea would be to maintain it like the vendor forum, with child boards for WTS, WTB, WTT etc... This will allow people with certain motivations to avoid sifting through thread titles to find what they want. If I'm looking to only see what people are selling, I can go to the one child board for WTS, and avoid threads of people looking for things or desiring trades.
A potential downside is one user having to maintain several threads instead of one, but I feel the user experience for other members and cleanliness of this method outweighs that downside.
My other main irk with the classifieds sub-forum relates to utilizing old threads for new items. You end up with multi-page threads with content unrelated to the subsequent new items that people have added to their thread. Rather than deleting the threads, I have to read the update dates, as well as sift through posts to see when the most current questions/answers begin. Some of the time, as mentioned previously in this thread, you have old sell threads bumped for strictly discussion that can take place in other areas.
A way to avoid such an instance is to cap time a thread can stay open. Let's say all classified threads have a 30-day limit, then they're deleted. If you don't sell all your items, or have new ones to add after the time period, you must re-list; not unlike some websites for classifieds or eBay.