In a store where products are shipped to customers from the warehouses of specific vendors, each vendor needs to specify their Ship-From address. The marketplace will use this address for the shipping costs calculation.
If no Ship-From address is specified, the marketplace will use the vendor's Company address (see Managing Vendor Profile Information as a Vendor) for shipping calculations.
A vendor can specify their Ship-From address via the Ship-From Address section (Store Setup > Shipping, Ship-From Address tab):
If the "Use your company address as your ship-from address" toggle in the Ship-From Address section is enabled, the vendor's company address defines the warehouse location. To specify a different address, the vendor needs to disable this toggle.
As a result, they will be provided with a form where they can specify their Ship-From address.