If you want to sell your book with our retail network such as Amazon, Alibris, Gardners etc. then you would require an ISBN and for this to be published and registered with the necessary ISBN registry (Nielsen, Bowker etc).
If you only want to sell on your own website, or sell through Bookvault's retail arm The Great British Book Shop, then you would not need to have a published ISBN and can use our provided Dummy ISBNs.