A staggering $1.6 billion worth of art changed hands during New York’s auction gigaweek last week. The three houses stuffed 11 sales of Impressionist, Modern, postwar and contemporary art into just five days. The combined total is up $400 million from the equivalent sales in May 2016, which made $1.2 billion. What should market-watchers take away from the onslaught? Here are our parting observations.