Private sales through auction houses are suddenly big, but are they new? “It was something that was kept under wraps at most of the houses for some time,” said David Schrader, who joined Sotheby’s last year as its head of private sales for contemporary art. “Now we’re being very vocal about it and putting more energy into it.”

There are compelling reasons for clients to sell far from the auction crowds. “The privacy is huge” as a factor, Mr. Schrader said in a telephone interview from Hong Kong, where he was overseeing the house’s first private-selling shows, timed to coincide with Art Basel Hong Kong. “A lot of people don’t want to see their work come up in a more public setting.”