What is the difference between buying my own ISBN and using one supplied to me by a publisher/service provider?
The ISBN identifies a book or other book-like product (such as an audiobook) in a specific format and edition, but an ISBN also identifies the publisher.
The purpose of the ISBN is to establish and identify one title or edition of a title from one specific publisher and is unique to that edition, allowing for more efficient marketing of products by booksellers, libraries, universities, wholesalers and distributors.
If an ISBN is obtained from a company other than the official ISBN Agency, the ISBN might not identify the publisher of the title accurately.
This can have implications for doing business in the publishing industry supply chain and can cause confusion and create difficulties with ordering etc.
Often you may find printers, designers or a publishing services company will offer you an ISBN as part of their service. However, these ISBNs will have the service provider—not you—recorded as the publisher and this will not be transferable to another publisher’s name.
A self-publisher should be identified as the publisher, therefore the self-publisher must get their own ISBN. (Quite often it also works out much cheaper.)