Get inspired
You could build anything with Goaddon. Feel free to read through our project ideas. Some could be usefull for your ecommerce company, others could be offered to ecommerce companies by you as a service provider.
What do you get from Goaddon?
By subscribing to the Goexplore addon you get a website that online shops can use to sign up.
By subscribing to the Gofetch addon you enable the merchants to import their products into your database.
If you install the explorer-at-goexplore repository, your website even get a products page that you can add the relevant buttons to.
What do you need to build on top of it?
So by now you have a website where a shop can sign up and enable the recurring import of products into your database.
You would need to integrate this website with an application of your own, one that can manage background processes. That application should recurringly check your database for products that has changed in stock count.
Whenever the availability of a product changes in the shop, your background process should perform the same change in the marketplaces. Either create a listing for it, update the available count or delete the listing.
How fast can you build it?
Integrating with marketplaces will require a few months of work.