If you are planning to self-publish your book through BookBaby, the answer is yes. Your ISBN is how we identify your book within our systems and ensure we are crediting all of your online sales to the correct account.
You can supply your own ISBN purchased through Bowker or another ISBN issuing agency from outside of the US. If you do not have an ISBN, BookBaby can assign you a number for each version of your book (eBook, printed book) for $49. You will need a separate ISBN for each format of your book.
If you are selling your printed book on your own (direct to reader sales from your website, book shows or book signings, etc.) you are not required to have an ISBN. However, if you wish to have any other type of distribution through online retailers, distributors or other catalogs, an ISBN is required.