No. It is not the same thing at all. The selling of an offensive weapon to anyone under age 18 in the UK is a crime. Possession of that weapon (in a public place) is a separate crime.
The selling of a car to anyone is not a crime. The use of that car on a public highway without compliance with the relevant laws is a crime. On private land no laws apply.
So in the first case both the seller and buyer can fall foul of the law. In the latter case only the buyer can.