Why does selling the data matter? If it's fine to collect but not sell it, people would already be fine with Google Ads tracking as Google is solely in possession of that data and will never sell it, lest their competitors gain an edge by out-header-bidding Google (offering higher profit margins to sites) with Google's own user data.

> Why does selling the data matter?

It is somewhat galling for your private behavior to be monetized.

But you're right that the selling itself is not the core issue. The core issue is that it's being shared. The monetization is an incentive to share, and that's where the problem lies.

By contrast, think about your doctor: they can't legally sell your private data. If they share it with anyone, it is for the express purpose of helping you, their patient. No problem there! My doctor shares my data with their lab testing partners and cloud vendor, and it doesn't bother me at all.

Now imagine if my doctor could legally sell my data to anyone and make even more money from me. We know with 100% certainty that every hospital would sell that data far and wide.

This is what adtech firms are doing, just with (slightly) less sensitive data than my doctor has.

> By contrast, think about your doctor: they can't legally sell your private data.

This is not a reasonable comparison. You choose to tell the doctor private information because of patient-doctor confidentiality. The other type of “private information” is collected by observing you in public (e.g. in a Walmart).

> You choose to tell the doctor private information because of patient-doctor confidentiality. The other type of “private information” is collected by observing you in public (e.g. in a Walmart).

I think this distinction is irrelevant. The salient questions are:

- Is this information private and potentially damaging to me?

- Do I expect [third party] to have access to it?

This is especially important if [third party] can be a government. In the massive web of interconnected buyers/sellers of adtech data, there is no reason to expect that oppressive governments will be unable to get anything they want.

But to address your point:

I absolutely do not expect "observing in a public place" to include personal conversations and/or data about exactly what products my eyes land on.

I'm pretty sure those big retailers are already doing this. Given sufficient profit motive, you should always assume the worst possible.