Help, please, understand

[barbadui]

Hello, please explain this point. At Sotheby's there is consumer interest. If the buyer is offered the highest bid so after the hammer blow he must still pay extra this is the consumer interest?
    For example, as stated on the site, Abramovich bought Triptych "for 86 261 000 $. This is the final amount paid to them or we need to count more consumer interest? If yes, then what? 12%of the amount?