My ads are free - but you could use it in conjunction with Ventrian Subscription (you can limit features in PA by Role) - check out my sign up page with the Subscription mod: http://www.pokerdiy.com/pokerdiy-pro.aspx

I also have mine setup so the ads are free but advanced features (such as extended run time and featured listing) are limited by role (paid subscription). Site isn't live yet, but it looks like it should work good that way.

Classified, by their very nature are difficult to charge for using a standard e-commerce cart because the financial transactions are conducted between the users themselves.

I'm actually looking at it from a different angle, where I'm charging for posting the ad, so the transaction is between me and the poster. Roles work for time-based subscriptions (i.e. post as many ads as you want for 30 days, etc.), but not for one-time posting.

That is one of the options I was considering also, but due to the lack of software to do that I chose the above-mentioned method.

On a related-note, Property Agent also supports a limit on number of items submitted based on Security Role (which can then be handled by subscriptions). Not exactly sure how that works, since I haven't tried it myself, if it is a limit of total items submitted or a limit on number of active items.