To go hand in hand with my other suggestion.
I run a large classifieds on one of my forums, once a deal is done the thread is locked and moved to a graveyard to stop any confusion as to what are sitll acitve deals, and to log any agreements that have taken place.
Would it be possible to have an option, which we can define on a per forum basis to let the thread starter to soft lock/unlock threads. This way, when a deal is done, the user can mark the thread "soft locked" and after X days it gets moved as per my other suggestion then "hard locked" (meaning an mod or higher has to move it back if needed)
Again simple options.
Allow Soft locking threads? Yes No
Hard lock after X days: