As a seller, the way you keep stock is how you ship stock. When two items cannot be equally exchanged and shipped in place of one another, then you need to "keep" them separately using a "stock keeping unit" dedicated to each grade or version of the product.
​

If you do not use separate SKUs for each version of products sold on your channel, and you have explicitly linked these variants to the same item SKU in SKULabs, then your shippers will need a way to distinguish which version of the item needs to be shipped.

Again, separate SKUs is the easiest way to maintain accuracy in this case, but if you are adamant about not configuring SKUs for each grade, the only indication on the order of which version was purchased is shown by the listing's title, which is located directly above the item's title on the order fulfillment page.
​

Another method is by making "grade" as in "new/refurb/used/etc" an "optional option" which would then show up as a line-item note / customization in red text on the order ensuring your team knows which unit to ship.
​