Apple ID Activation

If a second-hand iOS device has been 'Activation locked' then only the previous owner can unlock it, either by providing you with the account ID and password, or by removing it from his list of devices (as he should have done before selling it) - please see http://support.apple.com/kb/ts4515

If you are unable to contact him to do this then I'm afraid you will not be able to use the device - there is no other way of unlocking it at all. Apple cannot unlock it for you.

You should if possible return it to wherever you bought it and ask for a refund as in this event the device is completely useless.

You need to contact the seller and ask him to remove this iPhone from his list of devices. Otherwise, your iPhone will be a useless brick. Get a refund for it if possible.

Find My iPhone Activation Lock: Removing a device from a previous owner’s account - Apple Support