Usernames at the very least, as online identifiers.

Art. 4 GDPR Definitions

For the purposes of this Regulation:

‘personal data’ means any information relating to an identified or identifiable natural person (‘data subject’); an identifiable natural person is one who can be identified, directly or indirectly, in particular by reference to an identifier such as a name, an identification number, location data, an online identifier or to one or more factors specific to the physical, physiological, genetic, mental, economic, cultural or social identity of that natural person;

And they don’t need to be sold, just retained. GDPR applies even if there is no payment anywhere, even to non-commercial entities.