You have been defrauded. Only the original owner of the device can rectify the situation. Sometimes, even that cannot happen. As an example, sometimes people will make an insurance claim on a device, receive a replacement device, but never return the original device. In such cases, that device is property of the insurance company, and is considered stolen. I suggest you contact the seller and demand a refund.
As above reply stated, you can't "unblock" a stolen phone.