Respectable auction houses are fully RESPONSIBILITY for the authenticity of works.
Here I have to hand the receipt of the acceptance of Upssalskogo auction. The seventh paragraph on the back is "RESPONSIBILITY for the mistake. It states that the seller held responsible for podlinnnost object, but an auction has the right to demand from the seller the entire amount received for the sale item in the auction, if it is proved that an object "error." The auction for its part, guarantees the authenticity of the subject and held responsible to the buyer.
Slashala that Sotheby's, for example, a couple of times gave money "back"
for a proven falshak. I do not know the truth, whether they require the money back from the seller or assume all costs.