[dbrgn]

I assume if you made a business where the user can sell their behavioral data for a service in return, and if the whole goal of the business was selling data in return for a service, then I would interpret the data collection to be strictly required for the business goal and thus legal under the GDPR (given explicit consent).

The official goal of Facebook is not "buying your data" but "providing a social network". Thus, targeted ads are not strictly necessary for providing that service.

[icebraining]

I think the data must be actually necessary for the thing you're providing to the user. So if you're paying cash in exchange for data, that wouldn't be allowed, because you don't need someone's data in order to give them cash.