What happens if it's not an online-based sale and you only have the manual contact details or they don't reply to your email? I don't see anything in the registration rules that compels you to make sure someone else does their end of it right, once you have notified the registry peoples that you've sold it and its no longer in your lawful ownership and possession it should come off your ticket - period (capital full stop).
This is the issue with the registry people being seemingly only concerned with 'possession' rather than both possession and lawful ownership, as has been noted in the real world things aren't always so black and white as they try to make us believe (shared safe between family members etc etc).
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