> you are not going to sell it.

I don't know why someone would posit this falsehood.

You are going to sell it to EVERYONE you can (legally and sometimes not). There is no ad-tech company that does not take this stance. You get information about Honda buyers, you're going to sell it to Ford, then take what you sold to Ford (exclusively) and sell a repackaged format of the data (summary) you sold to Ford and sell that to Subaru ALONG with the data Ford asked for in the future.

Data management platforms (DMPs) are an entire concept built around selling the data. Every adtech company (that has any tech) sells the data that is collected, outside of using it for internal targeting. The number of demand partners and amount of useful targeting data to leverage for them, is necessarily a smaller set than what you collect.

Edit: This is experience from multiple senior ad-tech positions.