If you list everyone's inventory with one-click ordering (assuming that order fulfillment is solved somehow), it is going to be an immediate race to the bottom on pricing, perhaps accelerated by arbitrageurs. Consumers will initially be happy, until service suffers, but vendors will be hit hard on margins.
One niche where I could see such an app succeeding: less-urban areas. In the past year or so, Amazon has neglected my parents' market in the college town of Blacksburg VA. My Dad broke his computer mouse over the weekend; while I could get one in hours via Amazon here in Seattle, the earliest delivery Amazon offered via Prime on Monday was Friday, with no option to pay more for faster delivery. I guarantee that there are hundreds of computer mice in existing store inventory within an hour's drive.