Excellent initiative!
Lots of PHP-coded classifieds software are floating out there but -none- of them is really built for serious business.
They're nice but any serious web media group will code its very own MySQL database and then add -lots- of highly value-added PHP queries and rich interaction pages to make the visitor feel everything is absolutely top notch.
For instance, very big classifieds systems should mimic ebay in the sense that the most important module, for the user, is the "myebay" zone. A kind of place where all the important stuff for every single user is clearly stated and updatable. Of course, this is just an example, among many, to show classifieds script available right can be perfected... a lot.
Make sure to keep us posted when your "alpha-release" gets out the door ; )
I wish you lots of success with your new script!