Self-service is great, if you add in some approval process. That is how Google and just about everyone else operates. You add your own ads, and they approve them, then they go live.
Without that process, it becomes too labor intensive to sell to the public. An approval process takes a page view and a button click, whereas managing the ads for the customer includes multiple page views, communication with the client, uploading banners or typing or copying & pasting ad code, etc.