There are no "free" Wordpresss classified themes but you can use Wordpress with one of these paid plugins to create a classified site: 1. Classipress http://www.appthemes.com/themes/classipress/ 2. Sitemile http://sitemile.com/products/wordpress-classified-ads-theme/ Wordpress is easy to use, SEO friendly and very stable (used on millions of sites) and constantly under development. You can enable/disable registered user posting, like you said in your post. Both plugins aren't expensive and have good support too. Both come with lifetime free updates.
I've already installed a plugin that is offering everything that a classified site should offer... but I am unable to find a theme that matches it, as a blog's theme is still installed.. that was offered by wordpress by default !
I find and know this site have some theme wordpress that you need. You can check at here. http://iluly.net/156