Welcome to Android Central! He really should have removed his Google account first, then did a factory reset. As it is right now, if you were to do a factory reset, you'd still have to enter the correct password for that Google account before going through the Setup Wizard again, because that's how Factory Reset Protection works.
Contact the seller and tell him that you can't use the phone unless he helps you log back in so that you can remove his Google account and then do a factory reset.