Invoices, as a legal document, can serve as a safety net for you as a seller. In many cases where a client refuses to pay, claims the order wasn't fully fulfilled or that they bought something else, an invoice can be the only proof you have. However, as useful as they might be, invoices can be a pain to prepare, because they contain a lot of information, all of which needs to be filled in. Unless you use a solution for your inventory management that can also generate and send invoices for eBay customers.