South Korean e-commerce companies are increasingly turning to Chinese platforms to help sellers tap rising demand for Korean products, offering a lower-cost route into China and other overseas markets without building local operations. 11street, one of South Korea’s major online marketplaces, recently opened a storefront on JD Worldwide, JD.com’s cross-border e-commerce platform, where about 700 million consumers have made at least one purchase over the past year. The store carries products from...