As long as the seller removes it from their account and does a factory reset on it there shouldn't be any problems for the new owner to create an account or add it to their account and use it. You do not transfer the subscription, you create or use your own account with its own subscription for the device.
I don't have experience with this or can't fully verify but I have seen references to where someone bought a device from an unknown like ebay where the device was still registered to an account. From what I recall Garmin cannot fix this for you. The original person that had the device must remove it from their account. So that said be careful who you buy from and make sure if there's issues you can return it or cover yourself.
Actually I forgot that I do have a little experience with the above. A long time ago a friend gave me a Forerunner watch. I could not register it with Garmin and use Garmin Connect with it until he removed it from his account.