Crikey!
I'd be very annoyed if I was the buyer. In fact I just wouldn't pay for it. All of that is explained in the link. The buyer is NOT legally liable to pay for it. The auction house would be, but as this was a publicity stunt, they won't be! It's just another in a long line of Banksy'isms.
And I doubt he'd be bothered at it being destroyed. So much of his early work was cleaned off, co sit was nasty graffiti.Some of his best work disappeared under high pressure hoses!
It's just more of the same old same old, isn't it?
Momentarily shocking, funny, has a message whilst also being fairly meaningless!