Well, there is the Building Code 433 ff. However, the customer must identify the defect. Sorry to say: You enter into a contract when you buy something (online). You might also want to read the terms and conditions. They are available in every common online shop. At the latest by the age of 16, one should know simple obligations of the purchasing process or at least engage with them. Those who use online shops, here as well. Of course, you don't have to know everything.. but many things can be understood with common sense, even without reading the terms and conditions.