Do you want real inventory control? In that case the inventory is not dependent on the customer 'checking out' it is dependent on your staff despatching the goods, that is when stock-on-hand need to be updated.
But I suppose you want stock-available so you don't over-sell. I will also add that back-order is an option when all stock-on-hand has been sold so you don't loose business. You don't have to tell the customer they are on back-order just so long as your re-stocking is efficient, but you do have to get it right so your reputation does not suffer.
You will then be in the area of re-order levels and lead-times. Getting those right is the key to reducing inventory costs without lost sales, getting them wrong is a short-cut to going out of business. Efficient inventory control is what has made companies like Toyota,Tesco and Walmart winners and the other losers.
1 To run any sort of inventory control database system you are going to need to use Transactions. That way when the orders table is updated the stock table is also updated, and any associated buying system is kept in-step.
Sorry to throw all this at you but inventory control is a large and complex domain and you are best served by going away and reading up on the basics so that you can choose which systems and processes you need to implement. The impact on a business's bottom line can not be over stressed so it pays to get it right.
