Logging in to a new account and removing old one on c71kw-400 streaming box

I sold a streaming box ( c71kw-400) on eBay for someone. The buyer is saying they aren't able to get logged into their account because the previous owner is still logged in. I did reset the device before selling it which I thought would wipe this out. How can the buyer get the old account removed and sign in to their own account?