The last time a sudden pop in $100 million-plus listings occurred was in 2007 and 2008, just before the housing crash. In 2008, at least four homes in the world listed for nine figures. Only one ended up selling for close to that. A mansion in Palm Beach owned by Donald Trump and listed for $100 million sold for $95 million. (Mr. Trump says it sold for $100 million.) A 103-room mansion in Surrey, England, called Updown Court, was listed for $138 million, but sold in 2011 for about $50 million. A log mansion planned for the Yellowstone Club in Montana, with a promised price of $155 million, was never built, and the land sold for $10 million.

Of course, anyone can slap a $100 million price tag on a home to get attention. Yet actual sales of nine-figure homes are rare, even in good times. Between 2011 and 2016, only 15 homes in the world have sold for $100 million or more, according to Christies, and five of those were in 2014.