See
http://turnitin.com for an example of a commercial service that does full plagiarism checking already. Licenses aren't cheap, but if I were opening a digital publishing service, I would regard this as a critical feature, to avoid liability-- and bad press. Sure, the lawyers may get you off with the "author signed a statement swearing it was original" method, but meanwhile you'll have spent tons of money and time in court and the image of your bookstore will have been tarnished. Safer to check every entry (perhaps even including those from "real" publishers -- remember Kaavya Viswanathan? (
article and
a more sympathetic view)