Well - license plates are generated by an automatic process (eg you would get L1 AAA, then L1 AAB etc for all 1 letter, 1-3 numbers then 1-3 letters, although now it's 2 letters, last 2 digits of year, 3 further letters) and in general you would get a plate with your car. The DVLA (or DVLC as it was) pulled a lot of plates - rude ones were not allowed, and ones which looked like they would make money were kept back. Various other companies have jumped on the bandwagon, buying likely ones from the DVLA to sell to folks with too much money. They keep calling me up to try and sell me RMA 150P, but it ain't gonna happen. Incidentally I've got a number plate which is P KOX (if you ignore the pesky numbers) as it was on the car when I bought it. I enquired into getting a normal one, but it was going to cost too much. I've got used to it now.

But basically - you can buy and sell plates, it just costs money (various registration fees etc) and is a PITA.