Ok now look at it from the vendor side. I'm running my store, I need to make $2500 - $25,000 a day to make payroll. I carry a bunch of products, I don't make them, I just resell them. This kid is offering me a free inventory management service, that's really cool since inventory turns is one of the ways I manage my business, but they offer it to everyone. Now stocking X doesn't make a lot of sense if everyone stocks it, or I use the App to look for things stores around me don't stock and I add some of those, except they do the same thing. So now we're carrying the same stuff and we're in a race to the bottom in terms of margin. If I put my price on the item then the fact that there is a wall mart 2 miles left of here and sells it for 1/2 what I do will be painfully apparent. I'll get very little 'exploratory' business because people won't even come in to check on something if they know I don't have that one thing they are looking for even if I do have things that they ARE looking for but forgot they needed. Pass.
The 'big brand' stores almost all have a 'find it at a store near you' and there is probably a week of Node.js and UX work to create an App that goes out and wiggles all of those search boxes and simulates what you want. It can't tell you that Bobs Sporting Goods has an umbrella but if you're in your car and it can get you within a couple of miles you're probably ok with it.
So to pursue something like this you have to make it at least marginally useful for the people who sell stuff. And to understand them you have to 'walk a mile' in their shoes. That is the hard part, getting honest feedback about what they are going through.
That said, if you do spend the time to find their problems there are easily a dozen $100M ideas lurking in there to be solved.