If you use any type of inventory software, it probably asks you for a SKU or stock-keeping unit for each item. SKUs are different than the UPC or the ISBN as they are assigned not simply to a specific title but to a specific ITEM. You can easily have two books with the same IBN… but two different SKUs because one is new and one is used.
Many programs will just generate a general code for it. However, it may be worth doing custom codes to streamline order pulling and packing. You can pack a lot of info into those short little codes so as soon as an order comes in from the internet, you know exactly what to do before even touching the book.
First off you need an individual number string to assign to a book. Consider how many books you list per month. If its less than 100, you can use two digits. Less than a thousand, 3 digits, less than 10,000 4 digits and so on.