In selling usage data:

Someone pays Google, gets your usage data (and can do with it whatever they want).

In selling targetted ads:

Someone pays Google and gives them a specification of what kind of people they'd like to see the ads, and Google uses the usage data they already have to select who to show the ad to.

At the end:

In the former case, Google and the third party both have your data.

In the latter case, Google has the data, and the third party does not.

Whether third parties see information about your online behaviour or not. Is that not clear?

The end goal is the same but the only party with your information is Google as opposed to a third party.

ahem https://developers.google.com/bid-manager/guides/data-transf...

I'm not sure what you think that page describes, but it's NOT a way to buy data about how a person uses a google product.

You see, I know exactly what this page describes ;-)