Selling an account is risky. I think if you buy an account and the seller have their channel to recover like gmail. I think there is possibility that your account will be banned. And that because of suspicious accessing the account specially changing the password.
You are right. Just opening an email on different IP is risky nowadays. One time ago I open my old email while on VPN, and the result is gmail banned my account and I have to verify and answer the security question which I already forgot.