The ISBN is the 10 to 13 number identification barcode number required on any book you hope to have retailers, including Amazon.com, sell. The ISBN is managed by a worldwide organization. The USA agent for barcode registration is an organization called Bowker, although there are other agents licensed to sell the codes.

Buying a registration of the number is not enough. You need to produce the barcode of the number as well. Some people have the equipment to produce a perfect bar code. You probably do not have the proper printers and such, so you'll want to buy the actual image of the barcode from your source for the number. This image is offered as an optional add-on sale.