Of course, I'm not a lawyer. According to what I read and know, there is only a legal relationship in Germany between the seller and the direct, next buyer. The manufacturer has no contract with the second buyer but only with his customer, the first buyer. Of course, many manufacturers still guarantee from the original proof of purchase, even if they might not have to. There is always a difference between what is required by law and what you are willing to voluntarily provide in extended customer service.