Official comment
@Leo > The previous owner simply needs to deactivate the license from his installation, but if he has not, then it won't be available for you to activate. If you are still having problems after he has deactivated it, he will need to file a support ticket here, where our license team can force the deactivation - but he will need to request it, we can't do it for you simply because you have the license key, the request has to be made by the original owner.