I'm surprised Amazon didn't tell you out and out what the problem was, rather than it 'might' be registered to someone else. I would have thought if you were able to give them the serial number, they could tell you for definite if it had been reported stolen or lost and was therefore disabled to stop anyone else registering it.
Did you speak the the ordinary Amazon CS or Kindle CS? These are the numbers to use:-
Amazon US
In the US: 1-866-321-8851
International: 1-206-266-0927.
Amazon UK
In the UK: 0800 496 2449
International: +44 203 356 6212
You can also get CS to call you back or email them – click on “Contact Us” on the right hand side of the main support page.
If you don't get any joy, ask to speak to a supervisor and keep going till you get a direct answer.
Did you speak the the ordinary Amazon CS or Kindle CS? These are the numbers to use:-
Amazon US
In the US: 1-866-321-8851
International: 1-206-266-0927.
Amazon UK
In the UK: 0800 496 2449
International: +44 203 356 6212
You can also get CS to call you back or email them – click on “Contact Us” on the right hand side of the main support page.
If you don't get any joy, ask to speak to a supervisor and keep going till you get a direct answer.