What is an art advisor, and what was the Salvator Mundi affair about?

An art advisor helps a collector choose, buy and sell works. That this is not always simply a service for a fee was shown by the case of the Russian billionaire Rybolovlev and the Swiss dealer Yves Bouvier, who sold him works at an enormous, opaque mark-up: he acquired Salvator Mundi for roughly 80 million dollars and resold it for more than a hundred million. The case exposed the darker side of private sales, and it is a good reason to keep track of whom you are paying, and for what, even with an advisor.