An ISBN, or International Standard Book Number is not necessary when you write and publish your eBook. However, if you do choose to get one you will be able to sell your eBook at places such as Amazon. These numbers are assigned according to the country you live in and can be obtained for less than 100 USD. If you purchase then in a lot of ten numbers, the cost will be much less for each one. There are several brokers and agencies that handle this process correctly. It is recommended that you purchase your ISBN number before you complete you eBook so that you will have everything in order when you are ready to make sales.