I see this sentiment mentioned a lot, but who actually pays for this data? Is there a company I can call that will buy my amorphous "user data"? What are their names?

One user is worthless, a few pennies. A million and things get interesting.

Say I have a database full of metrics, user agents, IPs, GPS, names, addresses, photos, etc. on a million users, who would I call to sell it? Genuinely curious.

Here's a list of approximately a hundred companies that acquire and sell data in various ways, some of whom would plausibly be interested in your hypothetical trove of data:

https://konsole.zendesk.com/hc/en-us/articles/217592967-Thir...

One part of CCPA is that data brokers have to register themselves with the Attorney General if they trade in the data of California residents. The registry is public. You could start there.

https://www.oag.ca.gov/data-brokers

You don't call them, they call you.

Spammers. Spammers that used to buy phone number records or email addresses or postal addresses.

I worked for one for a brief time ~15 years ago (at the time it wasn't obvious what they were doing).

It's the kind of people that would have sent mass mail junk 60 years ago (and probably still send it today!).

Heck, you can see them even on this site, check for some marketing optimization technique articles that sometimes pop up here.

Even a good chunk of our fellow HNers are spammers.

Just because you haven’t heard of them doesn’t mean they don’t exist.

Ask yourself (as the article does): why would they even need this data? (menstrual cycle?? Really!?!)

Well menstrual cicle is a gold mine. But I miss the connection with headphones...

The obvious reason is money and a lack of ethics, but the dogwhistle reason is that these headphones are supposed to be used with a health and fitness app.