To the OP, as lizmonster says plenty of authors have no pub credits, and their books get picked up just fine. Credits help, but likely won't make much of a difference unless you have a good query letter already for what looks like a good book.
For the more recent posts, as someone who's had more success at short stories than with novels (and just to be clear: my success has consisted of half a dozen short stories in non-paying or token-paying markets, and two novels, neither of which sold more than 20 copies) I agree with Kinokonoronin that if we write short stories we should write them to write short stories. They're a perfectly legitimate way to tell stories, with their own craft in execution. I chafe a little bit whenever someone writes off (puns intended) short stories as a way to "practice" for novels--the false implication being that novels are the only true form of writing. Neither of you have said this, but I hear it sometimes. Novels too are a legitimate way to tell stories, and require a related but different set of skills to produce. Some skills are transferrable, but some are unique to each medium.
If you practice writing by writing short stories, you will primarily get better at writing short stories. If you practice writing by writing novels, you will primarily get better at writing novels. It's kinda zen that way.