Yes, you can sell books on Leanpub for free. If you want to, you can just set the Minimum price to $0.00, or you can set both the Minimum price and the Suggested price to $0.00.

Here are some details about how our "Variable Pricing" system works:

http://help.leanpub.com/en/articles/110750-why-do-i-see-a-suggested-price-and-a-minimum-price-when-i-buy-a-leanpub-book-what-is-variable-pricing

Please note that even if you set both the Minimum price and the Suggested price to $0.00, people can still choose to pay you money for the book (yes, on Leanpub people do often choose to pay money for books they can get for free).