I'm creating a fast, scalable, elegant solution (elegant in terms of coding) on top of OSClass: OpenSourceClassifieds. More info here http://github.com/opensourceclassifieds/core and here http://www.opensourceclassifieds.org Integration with Apache Solr, memcache, Redis, S3 Cloud front out of the box (all of them optional).
A simple but powerful way to build a classified site is using Wordpress. You can use Wordpress together with one of these plugins to create a great 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.
Few open source classifieds are there, but they are not so good to run classifieds business. I would recommend you to try AJ Classifieds Ads script for professional classifieds site business.
There is not such a thing as Good Open Source Classifieds Script. The script is good when no one has it and it is only you having it. Once its on internet and thousands of people are using it, it is over.
You can use Wordpress together with one of these plugins to create a classified site: 1. Classipress http://www.appthemes.com/themes/classipress/ 2. Sitemile http://sitemile.com/products/wordpress-classified-ads-theme/ Both have all requirements you mentioned. Besides that, Wordpress is easy to use, SEO friendly and very stable (used on millions of sites) and constantly under development.