Backstory: Kickstarter does a great job of building a sales page to get people to back products. Imagine if that same sales page was used to sell an available product?
The Idea: Kickstarter like sales platform for available products.
Deep Dive: Video's sell. Every element on the kickstarter page does an excellent job of convincing you to back the product. This is the same psychology used to sell an actual product.
If you combined all the same elements and allowed users to sell existing products, you would have a compelling sales platform in a great niche space. One key aspect I always view in any kickstarter project is the comments.
You could have a universal login for the platform allowing users to have order management in one place, their card would already be on file for "one-click" purchases and they would probably be more likely to comment once they already have an account created.
Validation: Amazon is a platform where you can sell your one off items but CrateJoy is the closest to this idea and has carved out a nice niche in the subscription box space. CrateJoy allows users to customize their own pages and sell on the CrateJoy platform.
Monetization: You could charge a subscription fee and/or take a percentage of sales. Going the sales route would be best since your revenue scales with your clients.