How is moving sold items to the bottom of the OP good enough? These sold items exist under the classifieds for as long as the thread remains there, which is literally forever. Potential buyers don't need to see these upon the search results. If you search for "sold" or "spf", many threads would come up.
How about making it so that users can post/edit threads in the sold section? When certain items are sold, they must archive them to their respective sold thread, instead of leaving them in their FS thread. When the entire FS thread is sold, they can mark it banana.