The following is from Garmin concerning transfer of ownership and registration:
If you purchase a device second hand from an individual and the unit is already registered to that person, you will see a message appear that states "The serial number you have entered has already been registered" when trying to register it to a separate myGarmin account. You will need to contact that person directly to have the device removed from their account so you can then register it to your own account. The original owner can simply delete the registered device from their account thus opening it up to be registered on a new account.
Below is a list of downloadable content or purchases that can be used if still loaded to the device, but cannot be accessed in the myGarmin account of the new owner:
◦Supplemental Map Downloads
◦Mad Maps
◦Safety Camera Subscriptions
◦Games
◦FM Traffic Subscriptions
If one of the above is already loaded to a device when it is received, the product can be used but no updates can be accessed from myGarmin without purchasing a new item, subscription, or service.
Because map updates (including the nuMaps Lifetime subscription) do not require registration, they may still be accessed if the device is sold or given away.
Lifetime updates can be accessed outside of myGarmin by connecting the device to a computer and visiting the map update page on Garmin's website:
http://www.garmin.com/mapupdates