I'd contact them - they've always been very helpful when I've done so in the past.
If you give the new owner your unlock codes and your customer code, there isn't a need for Garmin to know, I suppose - he/she will be able to buy upgrades etc, which is all the registration allows you to do.
Just re-register your new unit (if you have one) and get a new customer code...
Garmin will only need to know change of ownership if you've registered the unit.
If you have just send an email to Garmin notifying them of change of ownership including names addresses etc along with GPS type and serial No. and give all current GPS unlock codes to the new owner.