Many shops do double list, this is true. However, I don't think its a solved problem. There are many people who do not want to shop on Amazon for their own reasons. There are also people who want to shop locally, and Amazon provides no mechanism to do so (that I'm aware of). There are also many smaller shops who simply cannot afford to list on Amazon, as there are considerable fees associated with running a successful business there. It was these smaller shops who we were initially building to serve, to provide a funnel for them.
Still, there were problems with our solution that if addressed may have provided a better market fit. If we had had more runway, we would have worked to address them, but that simply wasn't in the cards.
To me it seems like a small market. And worse, it's hard to conquer that small market since it's very fragmented. Even if you had money for advertisements, it still would have been hard.
On the plus side, though, if you had the skills to build that platform, you certainly have the skill to build a more profitable and easier to monetize platform.
