Ryan Mahan grew up at the racetrack, helping his stepfather, a veterinarian, perform surgery on racehorses. He wanted to follow his stepfather’s career path, but Mr. Mahan was instead lured into the auction business. He started by selling everything from used furniture to livestock. He now conducts horse auctions for companies like Keeneland Thoroughbred Racing & Sales and the Ocala Breeders’ Sales Co., two major U.S. auction houses of thoroughbred horses in the U.S., and he has sold a number of Kentucky Derby winners including Big Brown in 2008 and one of this year’s favorites, Lookin at Lucky.