How to list a different inventory quantity per marketplace when NOT using FBA?

This article seems to suggest that I can create a “Unique Offer” in a new marketplace, working from an existing detail page from my home market. I.e. I have an offer and detail page for a product in the United Kingdom, and I want to create a “Unique Offer” in France for this same product.

However, this seems to rely on using the “Sell yours” button, which I cannot see. I just see a “Sell this product”, and when on the next page there appears to be no option to choose a “Unique Offer” or “Existing Offer” which contradicts what the article linked above says.

I need to make sure the inventory quantity and price can be set differently in each marketplace as they are fulfilled by two different warehouses. How on earth do I do this?!