The holiday shoppers staring into the display cases at a well-known upscale jewelry store in downtown Seattle look the part: Dapper-dressed gentlemen at ease with spending $26,000 for a diamond-pendant necklace.

Not so Mark Vadon. Youngish, stubble-faced and wearing cords and funky sneakers, Vadon appears stunned when the saleswoman hits him with the necklace's price. "The markup is insane," he said moments later outside the store. Vadon is not just a tough customer, he's also a shrewd competitor.

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