When Mr. Cui, an entrepreneur in the southeastern Chinese city of Hangzhou, wanted to draw more attention to the hair clips and costume jewelry he sold on the shopping sites of e-commerce giant Alibaba Group Holding Ltd., he says he turned to fake orders.

Faking orders, or “brushing,” as it is called in China, involves paying people to pretend to be customers. It lets vendors pad their sales figures and, in theory, boost their standing on online marketplaces, which often give more prominence to high-volume sellers with good...