When a customer buys one or several listings from one specific merchant on Back Market, an order is created. An order is composed of one or several orderlines. An orderline is defined by a quantity of a specific listing from an order placed by a customer to a specific merchant (orderline contains also specific information about the customer: billing, shipping addresses, etc.).
For example, if a customer buys 2 different listings owned by the merchant X and 4 same listings of the merchant Y, 2 orders will be created, one related to the merchant X and composed of 2 orderlines and one related to the merchant Y and composed of 1 orderline.
Actually, merchants only have to manage orderlines. Orders are treated automatically when corresponding orderlines are treated.
Note that in most cases, an order is composed of only one orderline.