Amazon.com Inc. opened to the public a cashierless store in the base of its Seattle headquarters about a year later than originally planned, the online retailer’s latest foray into the physical world. Shoppers are tracked throughout the location and charged when they pick out an item, eliminating the need for checkout counters. Amazon says it’s trained the system to recognize subtle differences in packaging and shoppers’ unpredictable movements.The technology has the potential to accelerate a shift already underway in the retail world, where brick-and-mortar chains are shifting employees and resources from the storefront to warehouses. Still, former Amazon executives say the Amazon Go store concept would be difficult to scale up to a larger footprint.