Hi,
It is not possible to use and not use the inventory system in the same time.
If you want the stock for your products, it means that, when a customer buy a product, the stock should be decrease.
But, if you always authorize to sell a product, even if a product do not have stock, it means that your stock would be negative.. So it would be a bug.
If you want a private value for your stock and not use the product, the best is to use a product custom field which would not be displayed for the customers (or displayed, if you want) but most important, which would not change automatically.
Regards,