> but I haven't seen any evidence that they sell data
It's worse than that, they give it away. RTB (real time bidding) on advertising works by presenting the bidder with a whole pile of info on the ad slot they are bidding on.
So even if you don't bid or you lose the bid you still get all that juicy data.
Interesting. Do you know a good place to read about the details?
OpenRTB specification?
https://www.iab.com/guidelines/real-time-bidding-rtb-project...
There seem to be three kinds of parties here, the advertisers, the exchanges, and the websites. An advertiser places targeted ads with exchanges. The websites send bids, potentially with user info, to the exchanges.
I'm finding web pages at Google showing that they run their own ad exchange, and they integrate with other people's ad exchanges.
I don't know how to find out if Google makes bids using any exchanges other their own, or whether the information from Google's bids could otherwise leak to advertisers.
There are lots of less well-known companies that will happily sell you lists, though, cobbled up from who knows where.