Buying an audience turned into buying cookie pools which evolved into real-time bidding, driving a publisher to pair the value of each user coming to their site with the advertiser that is willing to pay the most to hit that impression. Intelligent companies like BlueKai have been built to package together cookies with similar traits and sell them as faceless pools of “people who have similar online behaviors” to potential advertisers.
This is the exact principle behind securitization: pool together similar assets (in this case people who exhibit a propensity for a given behavior) and package them into one big bag of potential returns and sell them to investors (in this case advertisers on the demand side, or publishers on the supply side). “Investors” buy these cookie pools on levels of confidence (ahem, tranches) that the cookie pool meets their perceived value. With a higher confidence, publishers are charging more to target their inventory (yay more revenue) and advertisers can pay a lesser risk across ad networks (yay more revenue for long tail pubs and a lower CPM overall). This leads to a world where publishers and advertisers are encouraged to build more and more of these cookie pools.