German bookseller problems

I have started selling om Amazon Germany. I have sent a fair amount over Christmas and have had 2 requests for refunds, both from the same address but different named buyers. When I google it, its a bookstore so maybe buying for customers. It seems strange that every order I have is from a different name, same address. 1 book was very expensive but cannot prove they received it and just dont want to spend their money. Any ideas on German dodgy customers or maybe dropshippers?