How does a new KDP writer supposed to market a book?

Because you cannot persuade readers to buy any book in which sentences do not make sense. This shouldn't be “does.” If you don't see the problem, that is a big problem.

D) that are presented with bad covers, bad descriptions, or ludicrously bad interior formatting.

D) Pay promotion services like Written Word Media to promote your book. Legitimate services will not accept your book for promotion if the presentation is crappy or the book is badly written.

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C) Persuade book reviewers on BookTok or Substack or someplace to recommend your book. If your book is unreadably terrible, this is probably impossible.

A) that are not actually books.

If your book is well presented, well written, and basically free of typos, then

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Or

E) Take a solid year to learn how to use Amazon or Facebook ads and be prepared to lose quite a bit of money as you figure it out.

Or

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C) that are unreadably terrible in any other way.

A) Build a following on social media, a following of people who will buy your books. Good luck with that. There is very little evidence that social media can translate to sales.

B) Build a newsletter mailing list of people who will buy your book because they trust you to write books they want to read. This is slow, but ideal.

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Are you planning to proofread your book to avoid sentences such as

How does a new KDP writer supposed to market a book?

You cannot effectively promote books

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B) that are filled to the brim with typos or errors.