It may not play out like you imagine and the reason is quite simple. Now in order to make changes to your 2FA details for OTP generation you need your password to apply changes and as a result the account buyer wouldn't want to risk leaving the old password. Aside that, even if he did, how sure would he be that the seller isn't logged in still?
That's a valid point, but they might not mind for the time being if the seller grants access to the password to the new buyer and the buyer activates OTP on the account even with the old password not yet changed same goes for the email: the new buyer can log out the account from one device, and it will automatically get the old owner logged out and will be required to log in with password + OTP next time they want to log in, which gives the new owner an upper hand. In another case, the seller can also just decide to sell together with the email attached to the account to save the new buyer the stress and suspicion changing it might cause in the future.
But on a serious note, this email change, password change, and account being woken up after a long time of inactivity are just a primary way to suspect a sold account and the first step. The other suspicious thing comes in when the new owner can no longer keep up with the type of posting style the old owner used. Those who have interacted with the account before it changed hands will definitely identify that.