How important are previous publishing credits when submitting a manuscript? And does it matter where you are published?
The most important thing is good writing. Every author has to be published first somewhere. If you've never published before, but your novel rocks, I want it.
Publishing credits help with marketing, but only if the current product is good and solid. Past sales only go so far.
As to where your publishing credits are from, as long as it's a legitimate, professional publication with a critical and discriminating selection process (as in, they don't publish everything that is submitted), it counts. Self-publishing only counts if you've sold a significant number of copies (say 4,000+ in a year).