Hidden admin on Facebook

I take it that he's changing stuff without permission?

If it was a part of the sales contract, then he is in breach (reminder, folks -- put in clauses about the running of the Facebook page or group when you buy a business these days). But even with no specific such provision, he is likely still in breach (although it may be harder to prove in court as yours could be the first such case a judge in your area ever sees).

So contact the seller. Tell him you are giving him a week to relinquish his admin rights with screenshot proof or you are suing him for breach of contract.

Contact a competent lawyer in your area who specializes in contact law or internet law and sue if you don't get satisfaction.

Yours could be a test case.

Cant you just close your current FB account and open another?

You must use a new facebook account