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I would like to self-publish an e-book, preferably not using any third-party site (Amazon or Apple). As I understand from previous discussions here, I do not require to use ISBN. Which is good, because I found it pretty expensive.

My question:

What are the advantages of having (or disadvantages of not having ) an ISBN? Do I loose anything?

Note my question is not what are the rules of using ISBN or if I must use it in my case (what is discussed in the other topic). If ISBN has no purpose beyond fulfilling a regulation and has no any business advantage it beyond mare administratio, please clearly say so either her on the other topic.