Cost of sales represents the cost of goods sold or services rendered during an accounting period.
Hence, for a retailer, cost of sales will be the sum of inventory at the start of the period and purchases during the period minus any closing inventory.
In case of a manufacturer however, cost of sales will also include production costs incurred in the manufacture of goods during a period such as the cost of direct labor, direct material consumption, depreciation of plant and machinery and factory overheads, etc.