The convention couldn’t sound less rock-and-roll — the National Association of Music Merchants Show. But when the doors open at the Anaheim Convention Center, people stream in to scour rows of Fenders, Les Pauls and the oddball, custom-built creations such as the 5-foot-4-inch mermaid guitar crafted of 15 kinds of wood. Standing in the center of the biggest, six-string candy store in the United States, you can almost believe all is well within the guitar world. Except if, like George Gruhn, you know better. The 71-year-old Nashville dealer has sold guitars to Eric Clapton, Neil Young, Paul McCartney and Taylor...