Assuming default privacy policy, any possible legal repercussions for selling a forum along with database?

[Mr Lucky]

If someone sells a forum as a going concern and all users on the database agreed to nothing other than the stock T & C and Privacy, could there be any legal issues?

I always assumed there is nothing in GDPR that implies it could be a problem and loads of forums are sold, but I imagine some users may get annoyed if a forum they were on changed hands. Do you need their consent?
Assuming default privacy policy, any possible legal repercussions for selling a forum along with database?

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