You need to contact previous owner..... Sounds like he/she had reset protection enabled.....only previous owner can access reset key as far as I know....without that it's a paperweight.....sorry
The previous owner of the phone needs to remove it from their account before someone else can use it. They can do this at http://account.microsoft.com/devices
When they remove it, they will receive the recovery key. They can send you the recovery key so you can unlock the phone.