What if they didn't need to trust the website operators because they only pay them if users click? When the user clicks, it goes through the ad company's domain with the Referrer who would be paid for it.
Most ad fraud is the referrer owner faking clicks on ads on their own site. To trust a click you need to trust that the ad was displayed to a legitimate user. That's the hard bit.
Ad companies could move to only paying when ads result in a sale, but that only works if there's a sale that can be tracked. If I click a BMW ad and then buy one in the showroom that's really hard to track.
