Note: Goodreads only hosts reviews of fanfictions, not the fanfictions themselves. eBooks are not available for sale or download on Goodreads.
Goodreads is the world’s largest website for readers and book recommendations. For the most part it is focused on traditionally published literature, but their guidelines on what is and is not a "book" specifically indicates that completed fanfiction is also allowed.
This Fimfiction group is intended to track stories on this site that have had entries made for them on Goodreads. Or, if you think a story should have a Goodreads entry and it doesn't currently, you can post a request here (or add it to the "To Add" folder) to suggest it. Or one can add it oneself, as anyone can sign up as a Goodreads user.