I agree with the others here.
- If money is your main goal, you'd be better off writing something with higher demand than a story collection by one unknown author. If you just love writing stories, then do it.
- Get most, if not all, of the stories published in various magazines, e-zines and anthologies first. Even if they're not ones with huge circulation, they're still getting your name out there to some of the right readers and a small percentage of those readers will buy the book.
- The collection needs additional editing, beyond what was done for each individual story.
Consider placement, with the strongest stories at the beginning and the end. That way readers are drawn in and then leave wanting more.
Arrange them with care otherwise, too. For example, if you have three stories dealing with zombies, either use sub-headings for different topics in the table of contents, or space them out throughout the book.
Check for things that might have been fine in the individual stories but might not be in the book as a whole. For example, if you happen to have a character named Bob in more than one story, readers might assume it's the same character making another appearance and be confused when the two Bobs have nothing in common. Also, we all tend to have our little writing tics but it can seem like annoying author intrusion if the same strange phrase shows up repeatedly, etc. If it does, you'd want it to be a deliberate choice, not because you didn't notice it. This is where beta readers come in.
- Consider giving a story away for free on Amazon and make sure it's one of the best ones. People just love freebies, and a small percentage of them will buy the book.
- The cover is very important for drawing readers in to take a second look at your book in the first place. You might want to post whatever you come up with on here and get opinions on it.
Good luck!