Tri Tang, a 25-year-old marketer, walked into a Best Buy Co. store in Sunnyvale, Calif., this past weekend and spotted the perfect gift for his girlfriend.

Last year, he might have just dropped the $184.85 Garmin global positioning system into his cart. This time, he took out his Android phone and typed the model number into an app that instantly compared the Best Buy price to those of other retailers. He found that he could get the same item on Amazon.com Inc.'s website for only $106.75, no shipping, no tax.

Mr....