SHENZHEN, China — Chinese vendor Yin Xinwei sometimes makes close to $1,400 a day selling low-priced pill boxes, barbecue spits and other items to U.S. online consumers.

The future of that business, and the bargain prices enjoyed by his American customers, is now in question amid a looming U.S. regulatory change aimed largely at two Chinese e-commerce platforms he sells on, Temu and Shein.

The change, which comes amid rising trade tensions between the world’s two largest economies, is…