There's this comment thread under a post about student textbooks and a few other threads in the same post about the same legal loophole: Commenters are saying that while it is illegal to distribute copies of an original book, it's okay to make a copy for personal use, keep it, and then give away or sell the original book as used, and then others can repeat this until everybody has a personal copy, and there's only one original in the wild. Is this right?
The commentators are just making stuff up when they say that you can freely infringe on copyright as long as it is for personal use. It is true that "personal infringers" are less likely to suffer the legal consequences of any infringement (partly because it's easier to avoid detection and partly because the hassle to award ratio involved in suing a personal infringer is too high). It's a misunderstanding of "fair use", based on the legally erroneous assumption that anything is okay until you make a business out of it.