For a few days in March, Christie’s Rockefeller Center galleries felt less like an auction house than a public exhibition of 20th-century mythmaking. Fans came to see Kurt Cobain’s guitar, Jerry Garcia’s “Tiger,” and John Lennon’s piano from “Sgt. Pepper” and Ringo Starr’s “Ed Sullivan” drum kit up close. Then the sales began, and what had been a museum-like display turned into something louder, f...