When working with "standard products" you will have the ability to set and manage inventory levels. When you initially add a product to the store, you will enter the starting inventory. As items are purchased, the starting inventory will be decreased.
For store items that do not included sub items, you can view inventory remaining for those products, within the Store module, on the Products tab.
For store items that include sub items (ie. size, color, etc.), you can view remaining inventory by clicking the hyper-link for the store item from the Products tab.