By Tobias Carroll When it comes to ethics and the sale of art, it’s not hard to navigate into complex territory. Art looted by the Nazis during World War II is an obvious red flag that calls out for restitution, for instance — but when you begin looking into art sold under duress by (for example) Jewish families looking to leave Europe quickly, that’s something for which it can be more difficult to find definitive proof. In the case of a Claude Monet painting heading to auction via Christie’s, the auction house’s own restitution department was involved in researching the issue. That comes via …