Do only sandwiches that are in the inventory exist, or are there sandwiches existing that are not yet in the inventory?
Only sandwiches in the inventory exist. The components of the sandwiches are selected by users, with the selected components committed to the "inventory" as "a sandwich." (technically, there is no unified sandwich, just a bunch of committed variables which go in the next unfilled slot of the inventory screen, which is a table.)
So it looks more like:
Slot 1 (occupied) - Bologna - Am. Cheese - Peppers - Mustard - Sell (enabled)
Slot 2 (empty) - "" - "" - "" - "" - Sell (disabled)
Slot 3 (empty) - "" - "" - "" - "" - Sell (disabled)
Slot 4 (empty) - "" - "" - "" - "" - Sell (disabled)
Slot 5 (occupied) - Mustard - Lettuce - Bacon - Mayo - Sell (enabled)
ETA: So I don't come off as a complete asshole, I tried what I thought up in OP and it didn't work.
ETA2: -And I think I responded to your question incorrectly. "Pre-made" sandwiches don't already exist, but they can come into existence. Gosh, this is a pretty high-level conversation about sandwiches...