When Chelsea Wang travelled to Seoul with two friends in late April, they barely visited any of the city’s palaces or tourist attractions.Instead, their three-and-a-half-day itinerary revolved around duty-free shopping, beauty treatments, hair salons and cosmetics stores.Wang arrived with a shopping list of her own: a backpack she estimated would cost at least 500 yuan ($70) less than in China, and a bottle of perfume at a similar discount.One of her friends, Wu, a 28-year-old white-collar...