Same for me. Not sure what the time frame is but it's to avoid the "are these still available?" posts 2 years later. I can safely say that if after 6 months a for sale hasn't been updated, it's no longer available. Also forces posters to update their listings.
I can see one thread that @TollKeeper posted in on Mar.3 2020 just because it's listed as the last post on the main forum page and it was on a craigslist listing in Jan that is gone.