While marksmen use laser sights to line up their targets, eBay is using Red Laser. That is the name of a popular smartphone application that eBay recently purchased . Red Laser was already going to cause enough disruption for retailers with a physical presence but, with eBay behind it, it could be a game changer.

Red Laser is based on an elegantly simple idea: It uses the camera in smartphones to scan the UPC barcode on a product. Next comes the hard part: Red Laser queries Amazon and other online retailers for the price of the product. It also uses Google product search to find stores nearby where the product is also available, and at what price. The user sees none of the complexity. He just knows that, within a few seconds, he gets a list on his phone of alternative sources and their prices.

Now, retailers would rather not have that kind of strict price competition, and they’ll find ways to fight back. Some will withhold their pricing information, to try to make the system less valuable, or will try to post lowball prices and then add charges when someone comes into their store. Clothing stores will use barcodes that are unique to their chains to limit comparisons with items at other chains. The impediments, however, will eventually disappear. As Red Laser and similar services expand their reach, stores will find they have to participate or they’ll lose too much business.

Now for the game changing part: eBay will use its considerable muscle to make sure the ability to compare prices becomes widespread. eBay adds another important wrinkle, too: Not only will bricks-and-mortar retailers have prices compared against their nearby rivals and low-cost online companies but will also face competition from the entire universe of eBay sellers. Consumers will not only look at prices for the exact product they’re considering but will also see information about what economists call “substitutes.” Consumers will see cameras, toasters, etc., that are of comparable quality to the one they are considering but that are, perhaps, on sale because someone is trying to unload some inventory on eBay.

To take that one step further, what if eBay gives its sellers access to the shopper making the Red Laser inquiry? For example, sellers could offer their wares at, say, 80% of a Red Laser scanned item’s price. eBay could enable a special “take-it-or-leave it” bidding mechanism for Red Laser shoppers who want to make sure they are getting the best possible price before putting an item in their shopping cart. These and other new information and selling options could steal shoppers away, at the very moment when retailers thought that they had the advantage — at the very point when the shopper is in the store and seriously contemplating making a purchase.

Basically, anything a retailer tries to sell will, via Red Laser and its new owner, be compared against every other possibility. The only real limits will be the consumer’s willingness to sift through all the information and his calculation about the costs of driving elsewhere or waiting for delivery, rather than just buying what he has in front of him. But, with the ease of Red Laser and at the right price, consumers might just be willing to wait.