Using the same inventory of another store in the marketplace
The inventory assigned to the marketplace will always be the one defined by the seller once the marketplace will always consult the available inventory on the seller area.
This inventory, therefore, may be the same used by the main store or another one specific and customized for a marketplace or for more than one, with different rules and prices setup.
Using the same inventory of another store
This option will be chosen if you want to configure the marketplace to use the same logistics configuration of the main store or of another marketplace.
As the marketplace does not demand a special term or price, it also does not need a new logistics configuration.
So, just link the sales policy already in use to the new marketplace.