Dealer's licence to sell one's own car?

I would like to air my disgust at what happened to my neighbour recently.

He decided to put a sticker on his car with his telephone number so that anybody interested in buying the vehicle would contact him. He now tells me he has been booked twice by the police because he was acting as a second-hand car dealer without a licence! The car was parked legally and had a valid road licence and relative insurance cover.

So according to some silly law or some obtuse interpretation of this law, if I display my telephone number on my car I am breaking the law but if I place a daily advert offering my car for sale on a national newspaper or on a website then I am not.

Since when has a person selling his own personal car needed a second-hand car dealer licence to sell his car?

I would understand it is against the spirit of the law to place a stock of cars on the road and offering them for sale at a profit but everyone knows that, today, if you have to sell your car it will be at a great loss whatever the make or condition you have lovingly kept it in.

I would appreciate if someone qualified can kindly advise those people who want to offer their cars for sale without breaking the law.