A Chinese company few Americans have heard of is gearing up for what could be the largest Internet IPO in U.S. history. The company, Beijing Jingdong Century Trading Co., runs 360buy.com, a fast-growing online-shopping site that sells a broad range of goods, mostly direct to consumers, much like Amazon.com Inc. This business-to-consumer part of China’s online shopping market is expected to expand more than fivefold to 650 billion yuan ($100 billion) over the next three years, according to Beijing-based research firm Analysys International.