If you use this method, you have to keep track of sales when they are made, on an item basis. Its usefulness depends on whether the device you use captures the information with a minimum of effort and chance for error. Such data processing may be more economical than maintaining the records manually. Even when electronic data processing is not used, cash register tape may be such a device for capturing the needed information. If a sales count is needed on only a few items, you could check the possibility of using your register to get the information.
Sales slips provide another device for recording the item sales.
Still another is the price ticket which can be detached when the item is sold. Whether the information comes from register tape, sales slips, or price tickets, it has to be related to your stocks, specifically to particular classes of merchandise. A drug store, for example, needs sales information on prescriptions, proprietaries, fountain, sundries, tobacco, confections, magazines, and toiletries. The best way is to summarize the information and post it daily to your stock records. One disadvantage is that errors can creep into the posting of the daily sales.