No. Leanpub does not sell public domain books ("CC0"), even for the original author. We explicitly forbid public domain books in section 6.4 of our Terms of Service: https://leanpub.com/terms

Leanpub DOES support SIX of the normal Creative Commons licenses (CC-BY, CC-BY-ND, CC-BY-SA, CC-BY-NC, CC-BY-NC-ND, CC-BY-NC-SA). So, if you were thinking of writing a public domain work, hopefully one of those licenses works instead :)

If you want to use Leanpub to write a public domain book, you can do so -- you just can't sell it in the Leanpub store.

Here's why we forbid public domain books in the Leanpub store:
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Our opinion about public domain books is that since everyone owns it,
then no one owns it, and therefore if we allowed public domain books, then there would be nothing stopping any random person from taking every one of your public domain books and also selling them -- even on Leanpub.

Since it costs money to create a Leanpub book, this would seem like a good thing for Leanpub.

However, this would be a terrible experience for Leanpub readers.  Leanpub is all about supporting in-progress publishing, or "lean publishing", hence "Leanpub".  Now, lean publishing contains a bunch of ideas, but one assumption it, and all forms of in-progress publishing, make is that there is one thing being published.  Public domain breaks this.
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Since that is something we don't want, the simple solution is to ban
all public domain books from the store -- even for the original
author.