In Magento 2 Marketplace Multi-Vendor module, the admin can restrict the categories seller-wise. For this navigate to Customers -> All Customers.
After clicking on the All customers menu option, the admin will see a list of customers. The admin will select the user(seller) for which the admin wants to restrict the categories.
Select the Assign Category and then select the categories which the admin wants to allow for the seller as per the above image.