need to have a second stock location available to only tagged customers

[samkvl]

We have trade customers and the general public and have two stock locations (trade and public) on all products.

We would like to restrict the trade customers to only be able to access stock in the trade location (using a customer tag). General Public can access public and then trade if no stock in public (as currently does in shopify fulfilment order)

Any ideas? 

Thanks