While millions line up to buy Apple Inc.’s new iPhone 4S, Israel Ganot is shopping for old iPhone 4 and 3GS handsets. Ganot is chief executive and co-founder of Boston’s Second Rotation Inc. whose Gazelle.com service lets owners of used electronic devices trade them in for cash. Gazelle then resells the devices-a process Ganot calls “re-commerce.’’ Boston Globe reporter Hiawatha Bray recently spoke with Ganot about the growing market for used gadgets.