The inventory control methods that PowerStore uses are very specific to a PowerStore implementation. The ideas behind it would work the same in an eCart site, but the implementation would need to be specific to your site.
I think it would be best for you to create your own for your current site based on your needs. It all starts with the db and having columns for the inventory. You then need to adjust your admin area to work with inventory for an item.
The front end part is all about checking the inventory when an item is added to the cart, and when the items quantity is updated. After a successful transaction you will need to update the inventory for the purchased items.