A few years ago, Mercedes-AMG showed off the wild G63 6×6—a G-Class with six wheels, three axles, and plenty of off-road chops. They’ve long been sold out after a run of 100 production vehicles, but one has surfaced for sale. Listed on eBay in the United States, this 2014 Mercedes-AMG G63 6×6 has only seen one owner and 3,797 miles, and it…