Blog post from self-pub.net [updated: replaced “Createspace” with “KindleKDP”]
IngramSpark offers a better wholesale discount option for their distribution than KindleKDP. With KindleKDP, you can choose their Standard Distribution which lists your book at Amazon only for a 40% wholesale discount, or their Expanded Distribution, which gets it in other online retailers in addition to Amazon for 60%. At IngramSpark, you can get that same distribution for a 30% wholesale discount. So, while IngramSpark’s wholesale discount is better, you might experience stock issues on Amazon, which can really hurt your sales. (see: Understanding Wholesale Discounts When Self-Publishing a Book)
My suggestion is to use IngramSpark to distribute your book to everywhere except Amazon and use KindleKDP to distribute to Amazon only.
How that works is you would set up an account at both and upload your book to both. I typically let the listing go live on Amazon through KindleKDP first, then release the title for distribution through IngramSpark. Amazon will see that they already have the title through KindleKDP and will not pick it up through Ingram. If your title is already on Amazon through IngramSpark, you can also submit through KindleKDP and the KindleKDP version will replace the one from IngramSpark.
Here is a step by step:
If your title is already on Amazon through IngramSpark, you can still do this using the steps below:
Hint: Add something to the copyright page that is different from the file you submit to IngramSpark and KindleKDP and make the book’s description slightly different on each. That way you can tell where a printed book came from and verify that KindleKDP is handling the Amazon listing.
Orders placed through Amazon will be handled through KindleKDP. Any orders placed through other online retailers will be handled by IngramSpark. You will get payments for the book orders from each of them.