A method and system for enabling reliable verification of items being purchased using a self checkout system. The self checkout system is equipped with a verification mark dispenser and verification mark detectors that respectively dispenses a mark on an item as the item is being scanned for purchasing and then verifies, by detecting the mark on the item, that the item being bagged (or passed through the tunnel) is the same item that was scanned. When a customer scans an item for purchase, the purchase transaction is processed and, if the item is successfully purchased (i.e., transaction confirmed), a mark (made of a non-reactive but detectable material or ink) is placed on the exterior packaging of the item. When the item is placed in the tunnel, the detector, sensitive to the activation of the specific marking and located at or in the tunnel, detects the marking on the item. If the activated marking is not detected by the detector, an alert condition is triggered indicating that an un-purchased product is being placed in the customer's sack.