Competition and Transparency in Digital Advertising Act This bill limits certain large digital advertising companies from owning multiple types of advertising exchanges or brokerages and imposes certain duties with respect to the interests of the customers of such brokerages. Advertising exchanges and brokerages generally facilitate advertisers and publishers in buying and selling advertising inventory through an automated bidding process. Specifically, companies with more than $20 billion in annual digital advertising revenue are prohibited from owning more than one type of service within the digital advertising marketplace. For example, a company, such as Google, may not own a digital advertising exchange and provide software that assists publishers of online advertisements in selling advertising space on their websites. Additionally, companies with more than $5 billion in annual digital advertising revenue that provide brokerage services to buyers or sellers of digital advertisements must act in the best interest of their brokerage customers. The bill also establishes transparency and privacy requirements for such brokerages. The bill provides for enforcement of these requirements by the Department of Justice, state attorneys general, and private right of action.