Large ad firms don't tend to sell, or 'sell' that data, for five reasons.

1. The data is your competitive advantage, and costs billions of dollars to gather. You would be insane to sell it to someone else.

2. Regulatory. GDPR will absolutely destroy you for doing this.

3. Technical. Most targeted advertising relies on low-latency pipelines, with a lag time from minutes, to, at most, days between events being recorded, and ad targeting engines reacting to the event. You aren't going to sit around waiting for some third-party integration to 'launder' the data for you (Because it won't actually help you against regulatory scrutiny).

4. Practical. Ask people who have actually worked on these systems. Building it that way would be a fucking nightmare, both technically, and for the compliance pain.

Or, if you don't believe anyone, get a job at a large ad firm, and see for yourself... That there isn't any such Rube Goldberg contraption.

5. Even if, against all judgement, all of this happened, a large firm can't keep this a secret for very long, there's way too many moving parts, way too many people involved, and someone on the periphery will whistleblow, and the firm will be in a mountain of legal shit.