There is no problem in the buyer insuring the car whilst the seller has Insurance on it. The seller can and probably will cancel their insurance when the car changes hands .
Ok - i was just wondering that if for whatever reason the buyer had to make a claim and there was two policies in force on the car ; whether there would be any problems , with the insurance company .
I seem to recall that one of the questions i saw on a insurance proposal form was ' is there any other policies in force on car '
In the event that both policies cover the matter for which a claim is being made they will usually share the costs. However, with motor insurance it is unlikely to occur because usually the vehicle is only covered for a particular individual to drive. So in the scenario you describe if the seller still has a policy in force it is unlikely to provide cover when the buyer is driving or is in charge of the vehicle