This isn't quite what you're looking for, but the service that I use to digitally fulfill orders for my do-it-yourself incorporation and limited liability company kits (which are ebooks) is www.payloadz.com
They've got a service where once you sign up for their service and upload your product descriptions, you can check and box and then let other people sell your books.
I don't know how well this works. I.e., haven't done it myself (although I always think I should try it.) In theory, however, it would seem to be just an online version of what manufacturers have always done with retailers... (Note: Payloadz was saying for a while that one should be able to get 25% of their sales from affiliates... so that sounds good. But it also suggests that you would not want to rely solely on other web retailers...)