The newest, fastest, longest-range Gulfstream jet, the $64.5 million G650, entered service in late 2012. It has proven a hit in the marketplace, but that means a long wait for buyers—as long as four years from purchase to delivery. This high demand has led to quick turnaround purchases on the secondary market. About three dozen of the jets have been delivered so far, and several new owners have already sold their jets, at a profit of up to $6 million. And while many owners are private individuals and corporati…