That's no worse than the situation with insurance, I'm sure collecting road tax via insurers has been considered before.
Sugar,
Legally property changes ownership when you hand over the cash, but the "Registered Keeper" is different to the owner - I guess that changes when you register as the new one, i.e. when the DVLA receive the V5c.
I can see that the odd person might try to get a refund on a car they're still using, but there's no advantage to paying road tax on a vehicle you don't use. I'd anticipate any difficulty will be for the seller if the buyer is slow in sending off their paperwork and payment, or SORN declaration.
There is a little bit of a grey area, but I see no reason why you can't buy a car and drive it away before you become the new registered keeper, liable for the road tax. Is that also not what insurers have always provided cover for?