Anyone done ‘classified ads’ with WP before?

I’ve just wasted *heaps* of time surfing the web for an open source, free classified ads script. Few have been worked on much in years, except Noah’s Classifieds, which isn’t completely free though.
Well it just stuck me — wait a minute you idiot! — you’ve just made a paypal plugin and a proximity locator plugin for WordPress… and WP comes with most of the other stuff you need to put together a nice Classified Ads app.
So here I am. Tired… And excited about the idea of hacking a script together tomorrow. Before I get started though, I wanted to ask: Has anyone done this already? Does anyone have any pointers, tips?
Many thanks,
Eoin