When police raided San Cheng's Beijing apartment late at night and accused him of illegally buying weapons, he was sure it was a mess.It's true that he had bought dozens of toy guns on the Alibaba group's trade website Taobao, as props for his company, which designs shooting games for smartphones. But the seemingly harmless replicas were so cheap and easy to buy, according to Cheng, that he thought owning them couldn't be a crime.Was wrong. Cheng, 47, a Taiwanese-American game designer, ended up spending three years in detention and in prison. There, he claims that he met about 20…