Publishing an ebook

Do I need to be authorised by a "body" such as the ISBN to write and publish an ebook, I thought I could just write and start my promotion. 

Tim wrote a great article regarding ebook publishing

Thanks for the link Nikolas, so the ISBN basically applies if I want to set up myself as an international publisher. I notice he says if you are doing just one book it wouldn't make any sense. What is the advantage is using ISBN if I am selling ebooks?

I guess none, but I can't really tell as I don't know much about this business.

There are no rules to publishing content in the for of a written book in paper or online. You can write as many books as you want and people can buy them if they choose. Certs like ISBN only help you look accredited. I recommend writing the book and spending your money on marketing, not certs.