All products sold on Back Market must be supplied with compatible accessories. As such, it is mandatory to sell all smartphones, for instance, with compatible earphones and a charger. After-sales service requests are most often caused by faulty, incompatible or missing accessories.
Remember to send all the information needed for the proper functioning of the accessories to customers, as well, so they do not require an after-sales service request at all. For example, if your headphones are to be used under a Bluetooth activation, then indicate so by placing a sticker on the headphone box.