Are there any works of literature originally written in Esperanto, of which non-Esperanto translations have sold well or (for free online works) are popular among readers? (In comparison to other works in the target language, whether also translated or originally written in that language)
The book Vojaĝo al Kazohinio was written by Sándor Szathmári in both Esperanto and Hungarian versions. There is some debate about which version is the original and which is the translation. Wikpedia suggests that he started in Hungarian but then rewrote it in Esperanto before finishing. In any case it is considered an important part of Esperanto literature and it appears in William Auld’s basic reading list of original Esperanto literature.
The book has had several editions in Hungarian and also an English translation in 1975. If you take a look at the reviews on Goodreads, it is quite clear that the book has quite a large non-Esperantist readership.
William Auld was nominated for the Nobel Prize for his original work in Esperanto. Whether the nomination was based on quality or quantity ...