There are seven sellers on that listing, so you have no idea which or whose motor you are getting. What is certain is you'll get the cheapest motor someone can find.
Amazon charge sellers 15%, and they'll be paying £3 to send it. That leaves around £9 for the seller to buy the motor and make profit on it.
You get what you pay for. Have a read of this article: Cheap Dyson Motors - 150 Reasons Not to Buy One