A security system for inventory automatically detects removal of inventory items from an area or areas protected with security sensors. A person removing the item then scans an identifying code of the item, such as its bar code or radio-frequency identification (“RFID”) tag, with a device such as a portable shopping assistant device; if the item is not scanned in an appropriate manner, such as within a particular a time period, then theft of the item may be suspected. Information for completing a purchase transaction can be sent from the portable shopping assistant device. In this manner, items can be purchased directly from the protected areas.