When Auctioneers Don’t Know or Can’t See Their Buyers

FILE — Claude Monet’s “Matinée sur la Seine” is auctioned off at Sotheby's in New York, on May 14, 2018. High-profile artists can still draw crowds of bidders, but online bidding has accounted for a full 25 percent of sales this year at Sotheby’s. (John Karsten Moran/The New York Times).