So I think I'm dealing with a buyer with buyer's remorse.
I sold my Galaxy S7 Edge on eBay after unlocking it and then doing a factory reset. As part of the phone setup (when you turn it on) Android asks for a google account to associate with the phone, but so far as I know that's optional and you can "skip". I recall that from before the reset and I had the same thing with my new phone. I've never heard anyone suggest that after a reset and when doing new setup that you have to put in the account that was previously used for the previous setup.
Can I get confirmation from some expert here that this buyer is spouting non-sense? I can't understand how a factor reset would leave the phone in anything but a clean state or that Android upon starting would require the entry or association of the phone during setup with any previous Google account.