Reply to Used X1 troubles

Hey guys, I recently picked up a used X1 with the case and was told by the seller that he had gone ahead and unregistered it for me before selling. When I go to register it though using the serial on the back, the NI website tells me it's been registered by another user. I called NI and they told me to contact the seller again and I have multiple times with no response back. Any ideas on what I can do to get my X1 registered to my account?