Another way to check for and dl LifeTimeMaps, regardless of if the device registered (and to whom), or if it's not registered:

Go to the Garmin site > Maps & Accessories > Update Maps > Automotive. You will then see this screen. To find out if the device is LM, or entitled to a NuMaps update, just fill in the serial number:

garminmapupdates

You could also just connect your nuvi. I prefer to put my GarminDevice.xml in a Garmin folder on an SD card, USB stick or a virtual SD. That last way is safer because if something goes wrong with the map dl there is no harm done to the device. You can check the downloaded map for errors with GMapTool and if ok copy it and any associated files just as gma and jcv over to your unit. This method doesn't require a garmin account either.

You can try to register the device if you want. If it's already registered, get back to the seller and ask for it to be de-registered as dasilvarsa said.