A large share of the ads displayed by digital publishers (e.g., newspapers and blogs) are sold via intermediaries (e.g., Google), that have large market power and reportedly allocate the ads in an opaque way. We study the incentives of an intermediary to disclose consumer information to advertisers when auctioning ad impressions. In turn, we study how disclosure aects the incentives of publishers to outsource the sale of their ads to an intermediary, and relate these incentives to the extent of consumer multi-homing, the competitiveness of advertising markets and the ability of platforms to prole consumers. We show that disclosing information that enables advertisers to optimize the allocation of ads on multi-homing consumers is protable to the intermediary only if advertising markets are suciently thick. Even when most consumers multi-home, the publishers may be worse o by outsourcing to the intermediary, in particular if they operate in thin advertising markets. Finally, we study how the intermediary responds to policies designed to enhance transparency or consumer privacy, and the implications of these policies for the online advertising market.