honestly, I dont see how it could work economically. The cost of the machine and whatnot to me would never be covered by how much stuff would be bought by folks. I am totally a bike guy, but it just doesnt seem feasible or realistic to me to be able to sustain itself.
I share some of your skepticism and have the impression that the vending machine and tools at our local Chevron station are not seeing much business. The station manager/owner? is a cycling enthusiast so I think he had it installed despite the risk that it wouldn't be profitable.
But I think there could be some locations where it would be feasible on strictly economic grounds. In our area there are many commuters and others who combine bicycling with trips on our local subway/elevated (BART) train system. So the major stations see a concentration of cyclists. Furthermore, cyclists in the San Francisco area would quite often be within a reasonable distance of a BART station when needing repair parts/tools so if they knew that these stations had such facilities they'd be likely to head over to the nearest one.