But what happen if the seller take back his account by proving the ownership of old address after sell the account to someone? Is it still possible right?
Please respond, I'm curious about this one.
No need to worry if the buyer have signed message from the seller when buying accounts, that would serve as proof of account transfer and he cant claim it back
Not necessarily. It's entirely possible that this will happen and if you read some threads in Meta it has happened before. Some scammers will sell their account then try claim it was hacked to get it back.