Locking threads automatically is something that is not built in to vBulletin, which is our site software.
There is, however, a plugin modification available which can do that, but that's not free. Also, I no longer have access to make that kind of modification. Aside from the the moderators/admins manually locking old threads, (which would be time consuming and a logistical nightmare) the plugin is the only solution. Possibly the new site owners have a copy of the mod and would be willing to install for us.
Personally, I do not see it as that big of a deal. Mostly when a noob hijacks an old thread someone will steer them in the right direction, or ask them to post a new thread with their particular issue.
Of course too, whenever one of the moderators notices an old thread he/she may opt to go ahead and close it manually.
Looking at the member list to see who has and hasn't logged in is not a reliable method of gauging who is active and who is not since many members do not login just to view posts.
I am a member of countless forums and rarely login to gleen information, but rather only when I have a need to post. Nine times out of ten, much like many other sites, the search function nets the desired result since the question has probably already posed and answered by others.
As for the classifieds section showing old items for sale, I would consider it common courtesy for the original poster to mark the item sold.