Which e-commerce innovation first allowed self-published authors to directly compete with traditionally published books in the mainstream marketplace?

The publishing industry has undergone dramatic changes with the rise of e-commerce platforms. Traditional bookstores have faced existential challenges while new digital marketplaces have transformed how authors reach readers and how literature is consumed. This trivia question explores a pivotal moment in the evolution of literary retail that changed the industry forever.

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Amazon's Kindle Direct Publishing (KDP)
Barnes & Noble's NOOK Press
Apple's iBooks Author
Kobo Writing Life