If you can't get into the system without knowing a password that was set by someone you don't know, chances are it was stolen property or a lost and found item. Take it to the Police and it may find its way to the original owner.
If you buy another one, make sure the seller creates a login for you and you set a password when they aren't looking. Then oversee them deleting their account and after that, you have to go into G-Mail using your account. Make sure Google shows e-mails addressed to you.
That way the tablet is legally yours.