Hi Christian,
My name is Anne, on the Guru team. Thanks so much for getting in touch with us on the forums!
- It is possible to set up custom fulfillment options on your store for each of your suppliers. You can find our how to do this here- https://docs.shopify.com/manual/settings/shipping/fulfillment-services#custom
- That would notify your suppliers of new orders, however it wouldn't manage fees and payments, and shipping rates would need to be taken at the checkout and would be general rates for your full store. This means that you would have to manage this external to your Shopify store, and may need the help of an expert to set this up- experts.shopify.com
- There is an amazing app in the app store that may work for you! Its called Multi Vendor Marketplace, and allows you to add vendors and track everything. You are welcome to contact them by clicking on Get Support in the app store and finding out how they may work for you- https://apps.shopify.com/multi-vendor-marketplace
I hope this helps!
Warm regards,
Anne :)