The very first step in Amazon Brand Registry is to tell them your US Trademark registration number. Without it, I'm unable to get through the Brand Registry website. And I can't find any email address for them - nor, any place to mention this to. That's why I posted that to my website's home page.

Total speculation here, but probably because common law trademark would have to be proven in a court of law. That is, he would have to sue the Chinese company, present evidence like newspaper clippings or testimony from customers, win, get a judgement, and present that to Amazon. Until then, Amazon looks in the USPTO database.