what keeps sales persons 2-5 from selling same said part to which there is no mas?
Validation and calculation on your form (not in the table).
You can show qoh as the beginning balance minus all outs + all ins and show (e.g.) 8. You don't allow a new order qty greater than 8 even though there might physically 20 of them on the shelf. Using these design principles, you can even "reserve" quantities - either of in stock items (say for work order planning if that were your business) or future deliveries, although that's starting to get complicated.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.