This question gets asked often. Activation lock is a feature that Apple added to iPhones to prevent stolen iPhones from being used and to dissuade people from buying stolen devices. The owner must remove the device from his account or it cannot be activated.
If the previous owner cannot be contacted and has not removed the device from his account, you have a useless iPhone and should return it for a refund. If the seller is not the original owner and will not refund the money, you might want to consider contacting the police, as there is a chance that you are in possession of stolen property.