In recent years, the competition between the world's two largest auction houses, Sotheby's and Christie's, has seemed at times like a bit of an unfair fight.

Sotheby's, which is publicly traded, has lost out to its privately held archrival for several headline-grabbing consignments. Last year, Christie's sold the collection of Peggy and David Rockefeller for US$835 million ($1.2 billion), the highest-grossing auction ever of a private collection. In 2017, Christie's sold Leonardo da Vinci's "Salvator Mundi" for US$450.3m, the highest auction price ever for a work of art.

Both auctions were underpinned by financial guarantees arranged by Christie's, which since