AgreeI'd say a month maybe 2. If someone is trying to sell something that desperately, and it hasn't actually been sold, they'll bump it as needed.
They won't be deleted. Just closed and the poster can reopen if wanted toI would suggest something similar, but different. Another forum, DendroBoard, has a warning notification on threads older than a set period, that says "This thread is (xyears) old, are sure you want to reply? You may not get a response." That could be another option, as opposed to deleting the threads outright (which should probably still be done every year or two.)