Dealers worldwide earned about 36 percent of their sales on average through local or international art fairs in 2012, an increase of 6 percentage points from 2010, according to the European Fine Art Foundation’s Art Market Report by Arts Economics, which surveyed 6,000 dealers.For some, the share is even higher: according to Mr. VeneKlasen, 75 percent of his sales 10 years ago were made in his galleries, but now nearly two-thirds of revenues are earned on the road.