I used to work as an editor at a small press magazine -- we turned down plenty of stories.
The thing about someone selling their first story or novel is that it only happens once, it's a single data point. If you go by faery_queen selling her first story easily, or by me selling my first novel to the second editor who saw it, you could be misled. Sometimes someone, by a trick of timing, gets lucky, but the experience of dozens of writers that I know is that selling your first story or novel is usually quite difficult.
Also, in SF/Fantasy the number of places you can sell a short story are fewer than the places you can sell a novel, and there are more people submitting short stories. You can work out the logic of that yourself. For books, anthologies are less commonly published than novels -- and a large number of those are reprint anthologies -- single author collections are even more rare, and single author collections by previously unpublished writers so rare that I have never seen one. Which is not to say that they don't exist, just that their rarity would indicate they are many times harder to sell than single stories or novels.
If I were in your position and had my heart set on selling a collection right out of the gate, I'd strongly consider self-publishing, or a publishing house that asks the author to share the expense.