Many writers make the mistake of thinking that once they've placed a story, it's done. But there are many publications that take reprints (unless your story is freely available online--but even then, some audio markets might be interested) because their audiences seldom overlap. Putting some time into marketing your already published stories to reprint markets makes sense in terms of both potentially increasing readership and in gathering validation for your eventual collection of short stories. If I pick up a collection in a bookstore and see that all the stories were previously published, that increases my assurance that least one editor liked each story, even if the collection is now self-published...if there are reprints in "best of" collections or other journals, that's again as good as a testimonial... I still might not buy the collection if I don't know the publications or they don't seem to be my genre, or whatever, but at least I know it's not first draft vanity self-publishing.