If your classifieds are categorized in any way then after a listing is expired I would always 301 redirect it up to the relevant category, but I think it would be more relevant to 301 it to the relevant category than trying to decipher a similar specific listing.  I would find a way to dynamically after a listing expires it implement a 301 redirect.  I don't believe canonicalization is going to help here unless you have other duplicate content issues with the site.  I would not worry about the size of an .htaccess file because it doesn't impact anything negatively.