“No one has ever backed out of a winning bid to make a political statement before, art experts say. … Christie’s, like all auction houses, screens potential bidders and checks their financial credentials. [Collector Cai Mingchao], though, may not have set off red flags because he has purchased at international auction before. He paid Sotheby’s $15 million for a Buddha sculpture from the Ming dynasty two years ago, and owns a smaller auction house in China.”