NEW YORK, Aug 16 (Reuters) - Sixteen people who witnessed a white supremacist kill 10 Blacks in a shooting last year at a Buffalo, New York grocery store have sued social media and firearms-related companies, to hold them liable for causing emotional trauma. The complaint filed on Tuesday in a state court in Buffalo names as defendants YouTube and Reddit, where the gunman Payton Gendron was allegedly radicalized through exposure to harmful content, and learned information to help carry out his attack. Also sued were three retailers--Mean Arms, Vintage Firearms and RMA Armament--that allegedly sold firearm equipment and body armor...