Can the skull and crossbones icon be copyrighted?
Easy answer: you didn’t copy his work because you’d never seen it before he sent it to you, therefore it is not an infringement and he would lose that case in the “laugh test” phase. However, a TRADEMARK can be infringed by innocently selling something with a brand (logo, name, etc) “likely to cause confusion” of customers, who may believe you are somehow related to the person already using the similar brand. In the US Patent and Trademark Office, you can search the national database for “skull and bones” and dozens of registered designs come up. Ask for the guy’s registration number and look it up, or ask when he first started using it on products similar to yours.
