Most American publishers will only consider a first edition of a book, and while British publishers are a little more flexible on this point at the moment, many are moving round to the American way of thinking.
In publishing, as in so many other areas, there can only ever be one first time. So the instant that you first publish your work you’ve used up your first publication rights: and as you can never have more than one first time at anything, you can’t ever get them back. This is true no matter how that publication is achieved: whether you publish through one of the big conglomerates like Random House, a tiny independent like Salt or Bluechrome (which are growing in stature and reputation every day), whether you self-publish or get to market through one of the many murky vanity presses which lurk on the periphery of the industry: your book has been published and those first rights are irretrievably gone.
Now, if those first rights are granted to a reputable publisher with good editorial skills and a solid sales and distribution system in place, it’s likely to receive a decent amount of promotion and get into a reasonable number of bookshops nationwide. You have a good chance of making a decent number of sales out of that first publication.
If you give those first rights to a vanity publisher or to a disreputable or ill-informed small press, or if you self-publish, you’re unlikely to make many sales at all to anyone other than your family and friends. Even if your book picks up some good quotes or reviews, it is still unlikely to sell in any great quantity. You might find yourself wondering why you didn’t persist in finding a more mainstream publisher to bring your book to the market. And so you start to submit it again, using those good reviews as a selling-point. But you’re in a very difficult position here.
Despite those favourable comments your book has received, you’re likely to have sold relatively few copies of it: you’ll be lucky to have sold one hundred. And although this is many more than most self-published books sell, it’s still not enough to impress the big publishers who will, generally, want to see much higher numbers before they consider publishing a second edition of your work.