HONG KONG — Mere minutes after Donald Trump was whisked away from his rally in Butler, Pennsylvania, following an assassination attempt Saturday, printing machines on the other side of the world were already churning out T-shirts bearing the image of the former president with his ear bloodied and his fist held high.

It was Sunday morning in China, where retailers had listed merchandise showing the now-famous image on the Alibaba-owned Taobao, one of China’s most popular e-commerce…