This iPhone is currently linked to an Apple ID

The previous owner needs to remove this device from his profile:

https://supportprofile.apple.com

There is NO other way to unlock this device. If he cannot or will not do so, then demand your money back.

What to do before selling or giving away your iPhone, iPad, or iPod touch

When the previous "owner" selling the phone doesn't know his own Apple ID and password, it almost certainly means the phone is stolen or lost.

Get your money back. If they refuse to give you your money back, take the phone to the police. It's likely stolen.

An Apple ID and password is used frequently on iPhones to purchase or download apps and music. If the previous owner doesn't know their Apple ID and password the phone is most likely stolen. Get your money back or contact the police.

Good to know it's not stolen. But you need that Apple ID and password.

Looks like you really need to return it back to the owner, who sold you device where settings were changed by someone else. I wonder if that is possible to steal phone, box and payment proof? The good part is if you do have payment proof from autorized reseller, you can contact apple and ask for unlock, but they will ask your payment proof, not someone else's. So if payment proof was "owner's" he is the one who needs to call Apple and somehow I am willing to bet he will not.

ver8nica wrote:

The settings were made by somebody else, because the owner knows nothing about technology. The phone is not stolen!! I have all the accesories, including the box and a payment proof (from the store).

If that is the case, the owner who didn't know his own Apple ID/password had very limited use of the divice, and it may have remained in the box with the receipt. It is possible to steal the box...

Then you need to get your money back because the original owner, whether through ignorance or just plain stupidity, has rendered the phone completely unusable. It's a brick. A paperweight. It will NEVER work as a phone again unless they can remember their Apple ID and password.