[Amazon founder Jeff Bezos] recognized that a customer coming to Amazon and looking for a book would not know that not finding it in a search meant nobody would have it. In the retail world, that signal usually meant “go shop for it in another store.” Bezos saw the benefit of having his searchers find the book that was no longer available and being provided the information that it was “out of print”. That would encourage the customer to find a replacement at Amazon rather than search other retailers for the unavailable book.
The article also points out that Amazon has an advantage over brick-and-mortar booksellers because Amazon (at least at founding) did not keep an inventory of books on-hand; instead, when customer orders came it, it passed them through to one of the two big book wholesalers (Bowker and Baker & Taylor), who did the actual fulfilment, warehousing, et al.
The whole publishing industry is kind of end-to-end messed up, and extremely conservative in the “we do it this way because we’ve always done it this way” (it’s a, roughly, five-hundred-year-old industry). Most times when people talk about “disrupting” industries it’s code for “loss-leading with VC money in a way we’re pretending isn’t totally illegal” (ref. Uber et al.), but with Amazon, it really was legit the real deal. Of course, the longer-term effects have been utterly disastrous to, conservatively, all global retail everywhere. But . . . y’know.