Oh dear, it seems that Antiguru doesn't understand the difference between being the owner of a vehicle and being its 'registered keeper'.
A V5C is not a certificate of ownership. For example many people, who drive company cars, will be named on the V5C as the registered keeper. However, the car remains the property of the employer (or of a leasing company) and the registered keeper is most definitely not permitted to sell it!
If you lend a car to someone, on a long term basis, it makes sense to transfer the registration document to them. This means that they'll be responsible for ensuring that it's taxed. Any speeding tickets will also be sent straight to their address, rather than to yours.
If the person then sells the vehicle, without your permission, they would definitely be breaking the law.
Chris