How can I stop end user to purchase inventory item from exteranl supplier?

How can I stop end user to purchase inventory item from exteranl supplier?

Create an

org 1 and linked to location A

org 2 and linked to location B

item X and assigned to org 1 as inventoy and puchasable  and internal and internable orderitem

item X and asigned to org 2 as non invenrtory and non purchasable non internable item

create two responsability internal and external

user has in his preferences location B

using external responsability

enter item x in iproc

add to cart

change location to location a and able to proceed

How can I stop end user to purhcase inventory item?