In Japan, license plates, called 'number plates' here, are never collected, nor do they remain on the vehicle when it changes ownership. The plates are returned to the legal authority, the 'Land Transportation Authority', who scrap them.
So no nonsense of paying premium prices for a registration number.
There is a way in recent years though to personalize your plates.
The characters at the top show the prefecture or the district where the vehicle is registered. Here this is Hiroshima, 2 characters

The only available option is the 4 digital numbers, xx - xx.
For my XM I chose 12-12, my wife's birthday

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Due to the size of the vehicle and the engine displacement it fits in the category '3' for taxation purposes. This number is shown after the characters 'Hiroshima' as shown above. Category '3' vehicles have been registered with a simple '3', '33' and '31', '32', '34', '341' and other combinations starting with '3' to cope with the deluge of new registrations.
The request for the combination 12-12 could be accommodated by a computer generated '3xx' number and an available hiragana syllabary prefix of which there are 40.