Five-time World Series winner and baseball Hall of Fame inductee Reggie Jackson opened a Triangle Honda dealership in January. But his personal pantheon of vehicles features some of the world’s rarest automobiles. Car connoisseurs with beefy budgets had the chance to buy an especially exclusive member of Jackson’s collection in an auction that closed Wednesday: a 1990 Ruf BTR III. The German supercar is a custom-built version of Porsche’s 930 Turbo. Its winning bid of $345,000 was paltry compared to previous price tags on other BTRs. A 1985 BTR III in Arizona sold on Sotheby’s for $445,000 in January. Jackson’s...