In fairness I don't see how they can make selling a number plate harder, as it's not illegal to own it - i.e. in your kitchen drawer, nor does it legally belong to anyone - I think the "owner" is actually just the person who has the right to choose which vehicle it goes on, but I'm sure someone will correct me if needed
There's lots of things you can buy without the authority to put to its regular use - police uniform/warrant card, military medals, dummy banknotes etc... all sold with a supposedly genuine reason but I'm sure often not used for that reason
My dad just before he left the police about 10 yrs ago said herb grinders were the big thing at that point, supposedly lots of people wanted to grind industrial amounts of herbs for cooking 😂