Private Marketplace (aka private auction, private exchange, closed auction) is a type of programmatic advertising. It refers to a closed real-time-bidding (RTB), invitation-only, auction environment for digital advertising that leverages the media owner's ad inventory, typically to a select number of advertisers. Inventory is bought and sold at an impression or price level and it is a one-on-one deal between media owner and buyer, facilitated through a private marketplace such as Criterio or Rubicon Project. It also allows media owners to monetize their remnant inventory and place rules around who can purchase impressions.
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