In June 2016 ismaa3 from El Campillo in Spain was selling this „Sealand Diplo license plate“ as used.
It is not a real plate, it is unused, Sealand does not have any cars and does not have any diplomats anywhere in the world as there is only one family living on an old fort in the North Sea off the east coast of England.
They were sold as souvenirs to generate money for the family to live. Now it is for sale as something it is not for a ridiculously high price for a small piece of Perspex (Plexiglass) and a plastic sticker.
Since the Ebay page says ‚country of manufacture: Spain‘, maybe he is now making these himself.
Buying plates like this means you will end up with rubbish in your collection.