Same in Poland,
How comes that people can sell plates to others in UK, though?
In the UK, cars are given a plate (assigned to that car) when they are first registered.
These plates generally have some letters and/or numbers to indicate car's age/origin and the rest of the letters/numbers are random. The patterns and meanings of plates also vary by age (e.g. 90s: X999XXX, 2000s: XX99XXX).
People can buy and sell number plates to each other and to reselling companies (the government also keeps some of the good ones to auction). When you buy a number plate you must attach it to a vehicle, and the vehicle must always be newer than when the number plate first came out.