My suggestion is to have a section where people can post a link to a thread that they have created, that they would like to have deleted from the forum. It would help to tidy things up a bit without the moderators having to troll through looking for expired cl***ified ads !!
Or just click REPORT on the original post (underneath the poster's name) with a note to delete the thread. It's the same way you report SPAM threads that need to be deleted.