I don't have time to go into the background to this, but it seems to me that, absent the art dealer and the investment trust misrepresenting the painting in some significant way, the entire liability should fall on Sotheby's, who received five per cent of over ten million dollars for authenticating the painting and who admitted liability by repaying the buyer every penny. The two sellers should, again in the absence of any positive misrepresentation from them, be covered by the doctrine of caveat emptor -- buyer beware. But in this case the buyer presumably relied on the expertise of Sotheby's, and paid a very substantial sum for it, so even caveat emptor comes to rest on Sotheby's, and therefore in this case Sotheby's by two lines of reasoning is the "someone" in the judge's quip "the law has to fall on someone".
Super painting, by the way; a truly characterful portrait. I wouldn't give a Picasso houseroom but I would hang this Hals painting, even if it were a "Hals", whoever painted it.
Connoisseur of quality, not brand names