Someday, some way, ClassiPress HAS to HAVE a "shopping cart" for ads. Otherwise, it will remain much too rudimentary a product for "serious" applications.
The sooner an intuitive and flexible shopping cart arrives that allows options to the Admin and the User the sooner David Cowgill shows up in the Top 100 Online Entrepreneurs of Forbes' magazine!
The leaders in shopping carts designed specifically for WordPress based applications appear to be these guys....
If this really is a "bolt-on" series of applications -- (they do have a Free "starter kit" that requires selecting Categories that you already have), perhaps it can be integrated or just "strapped on" to ClassiPress.
And maybe it only takes some modding to make it truly fit.
However, I am going to look at this in a testing mode in the next few days.