I have two Autel devices in my device list and I just sold one of them. I thought I saw somewhere a long time ago an option to remove device from the list but I can't find it anymore.
I guess registration only gets removed and transferred to new owner if the new owner buys a new update for it, so in the mean time the new owner of your old MaxiSys you sold him can surf the the net and troll folk as 737 mechanic :-)