Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for receiving, from a client device, a first notification of an ad space from a seller, identifying ad conditions corresponding to an ad space inventory to which the ad space belongs comprising a first condition that identifies an allowed creative media type for the ad space inventory, sending a second notification requesting a bid on the ad space to bidders, each bidder representing a respective buyer, receiving bids from the bidders, each bid corresponding to a respective bid price, buyer, and a creative, determining that a first bid of the bids corresponds to a first creative having a media type different from the allowed creative media type, and identifying a preexisting arrangement between the buyer corresponding to the first bid and the seller and, based thereon, allowing the first creative to be served to the ad space.