We feel your pain. Conscientious sellers come back and post a sold
notice, and moderators move the ad to the archives. But many forget, move on to other things, or simply don't realize the responsibility they have under our marketplace rules. They may assume ads automatically expire after a certain time like other sites.
If you click on the seller's user name and check out their profile, you should be able to identify the date of their "last activity" on the forum. If they haven't visited the site since three days after the ad posted four months ago, you can bet it's gone.
Best wishes finding your trailer! It will come...