Stock Management

Hi

I have realised that I need some kind of stock management.

When the admin adds a product, they submit a quantity of stock in which they have to sell.

What I need to make my store do is reduce the quantity of stock each time a product is purchased.

For example, 14 should become 13 after purchase is complete.

In addition I am not sure how it would work on the product detail pages. If an items stock reaches 0, then the item should not be purchasable.

In the same way, if a products stock field is null then it has unlimited stock, and the Stock Management is not used. (Mainly For Digital Goods)

The last question I have is, if an item has a stock of 1, what happens if a user increases the quantity of the product to 2 or higher on the cart page.

Thanks...