If you keep it around to sell then you are likely violating the 'legal basis for processing' part of the GDPR. Data can only be used for the purpose for which it was originally collected, selling the data to others to use without that exact same goal can not be such a purpose, and even then you will have to be quite careful that you maintain control. Various EU data brokers (Schober, for instance) have found ways to do this in a controlled manner usually by anonymizing the data or by selling it only in aggregate form.

But selling it raw with the personal identifying information of the data subject is almost always a complete no-go.

It means that democratic societies have decided that that type of business practice is undesirable, and should go away.

I guess the implication is you should market like you would for electricity, that is, metered subscriptions only.