The Mariana Tek does NOT allow for a negative amount of inventory to be sold. If a product's inventory levels reach 0, customers and admins alike will not be able to add the product to the cart. Instead, they will see an error message notifying the user that "there is not enough inventory to fulfill this transaction."
To avoid this, you will want to keep inventory numbers updated or you could choose not to track inventory for some items, therefore allowing you to sell the item as many times as you want. You can adjust whether or not to track inventory in the product configuration setup.