Perusing a library's used book sale, I found myself torn between buying copies of older, classic texts, and also knowing that a) I wouldn't likely read them (but might), and b) they were probably online, at Project Gutenberg. So even if I found a need for them, I could get a downloadable and searchable copy online, for free.
The internet has become a vast library and storage system. Not the best versions, mind you--without scholarly corrections and footnotes. But if you've wanted to read The Mayor of Casterbridge on a casual basis, why not grab a free copy off the web? It can be transferred to an iPad or Kindle or Nook, and zoom you're off. Before, book sales were the best way to get cheap copies of books you wanted on the shelf, just in case. Now, with the internet, you have immense resources just a few links away.
Another distinguishing feature is the searchability of texts on the web. I found this quite useful in writing my dissertation on Thomas Hardy; if a passage was in my memory but not easily found, I could search for a word or two and find it quite instantly. I could also search for some interesting words, such as "cruelty" (one of Hardy's great admonishments, to avoid cruelty to all creatures), and see that it occurs so many times within a certain text. I wouldn't cite that research in my scholarly work without checking a corrected text thoroughly, but it still gives one some interesting points of research and ideas.
So, to buy books you're only vaguely interested at a used book sale, or not? I walked away with 10 books after all, since they were cheap and of possible value to me. Now I have to figure out where to store all these books, and even begin the process of weeding out some as donations to other book sales … And so the circle of life, err, books, continues, only with the internet now providing an interesting inner loop.