> Even the GDPR gives us enormous leeway to do literally this, but it requires participating in tracking networks that have what amounts to a total knowledge of purchases and browsing you do on the internet. That's the only way they work at all.

That data sounds like it would be very valuable.

But I think if I sell widgets and a prospective customer browsers my site, telling my competitors (via a data broker) that customer is in the market for widgets is not a smart move.

How do such tracking networks get the cooperation of retailers, when it’s against the retailers interests to have their customers tracked?

They get demographic data on their customers and can use that for marketing and setting prices.