Christie’s and Bonhams have both quietly raised the fees they charge buyers. The charges, which auction houses like to call “buyer’s premiums,” are added on top of a lot’s hammer price and are a key source of revenue for the houses. At Christie’s, effective September 1, the house is charging 28% of the hammer price of each lot up to and including £1.5 million or $2 million; 22% of the hammer price from £1.5 million or $2 million up