The line between auction houses and art galleries keeps blurring. It is no longer unusual for Sotheby's and Christie's to take over entire galleries. Sotheby's did it in 1996, when it bought the Andre Emmerich Gallery. Christie's followed, taking over Hall & Knight in 2004 and then Haunch of Venison three years ago. Like dealers, both companies also do much business selling works of art for clients privately.