[DiapDealer]

The only thing I'm going to say is: the Way Back Machine can't "sell" anyone a book. As such, they can't be guilty of copyright infringement or piracy as some are suggesting. They're archiving the front-end of the store only. They're not archiving the database back-end or the payment processing back-end, or the ebook delivery back-end of a retailer's point-of-sale system. At some point, a link from the Way Back Machine is taking the user back to the retailer's current website to complete the "sale" of a free book. Which means urls still exist on the "real" site that allow the free sale of a book that was previously—but no longer—free. So those urls could be used with, or without the Way Back Machine being involved.

Thus there is no valid "it's the Way Back Machine's bad" argument.

Given the rareness of the parameters (a book you need to complete a series that you CAN'T get by ANY legal means), I vote "don't care." Anything else is cheating, and falls squarely on the shoulders of the person doing it to own up to it (or to pretend it's justified).