[mikejb]

Unfortunately none of your links referred to the actual legislation and only provide summaries and interpretations, but Article 17 (Right to erasure), Section 2 states:

Where the controller has made the personal data public and is obliged pursuant to paragraph 1 to erase the personal data, the controller, taking account of available technology and the cost of implementation, shall take reasonable steps, including technical measures, to inform controllers which are processing the personal data that the data subject has requested the erasure by such controllers of any links to, or copy or replication of, those personal data.

So there's an obligation to inform. Which makes sense: Assume you (with consent of the user) sell data to some company, you have to forward them the request of said user to be forgotten. But you have no authority over that company. You can report them to any and every authority if they ignore or actively refuse your request, but you as a company have no authority over them. Particularly if they don't operate within the reach of the European authorities, there's little you can do. And that's what I mean with "You surrender control of the data once you sell it".