Plus, bidders get to observe billions of auctions; many thousands from each user. Across all auctions, we can identify common users, and track them over time, b

Plus, bidders get to observe billions of auctions; many thousands from each user. Across all auctions, we can identify common users, and track them over time, building an increasingly accurate (and valuable) behavioural portrait of each user over many months of browsing and clicking. You can also associate the location data that you get from mobile phones, enabling you to tie in to old-school demographics databases, as well as build a social network of co-located individuals to augment the targeting information obtained from other social networking sites. (Not to mention the behavioural profiling gleaned from the location data itself).