Sara FischerData: GroupM; Note: Apple and Microsoft values are estimates; Chart: Axios VisualsApple has started selling its own advertising inventory for Apple News, two sources familiar with the effort told Axios. It's pitching new ad units that it hopes will maximize revenue for itself and its publishing partners.Why it matters: The shift toward direct sales represents a significant milestone in Apple's advertising ambitions.Apple has been selling its own ad inventory in search and within its ...