1. A method for providing sales information using an information providing system, the information providing system including a database, a display, a detector, a first motion detector, and a server connected to the database, the display, the detector, and the first motion detector for exchanging information there between and controlling the operation of the information providing system, the method comprising: detecting, using the detector, a change in weight on a display shelf based on the movement of a commercial item with respect to the display shelf; detecting, using the first motion detector, the movement of a person proximate to the display shelf; receiving, using the server, information from the detector regarding the change in weight and receiving information from the first motion detector regarding the movement of the person; obtaining, using the server, sales promotion information stored in the database and controlling the display of the sales promotion information based on the information received from the detector and the first motion detector; displaying, using the display, first sales promotion information obtained from the database when the detector has detected that the commercial item has been picked up from a display shelf based on the change in weight on the display shelf; displaying, using the display, second sales promotion information obtained from the database, which is different from the first sales promotion information, when the detector has detected that the picked-up item has been returned to the display shelf based on the change in weight on the display shelf or when the first motion detector is detecting that a person is present around the display shelf; and measuring time taken until the commercial item is returned to the display shelf after the commercial item is picked up from the display shelf, wherein the second sales promotion information includes at least two types of sales promotion information, when the measured time is within a certain period of time, second sales promotion information of a first type is displayed on the display, and when the measured time exceeds the certain period of time, second sales promotion information of a second type is displayed on the display.