buy a i cloud locked phone

Hello ~ You need the original owner to unlock it. If you cannot contact them …we cannot help and neither can Apple. Return it for a refund or recycle it responsibly. Take a look here:

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Also:

The All Too Common SAD Reality of Buying a Used iPad/iPhone

~Katana-San~

Only the owner of the account used to lock it can remove the activation lock.

The phone you purchased is useless. If the seller can't or won't remove the lock, return it for a refund. If the seller refuses a refund, turn it over to the police. It is most likely stolen.