Used book stores--I'm personally I'm glad some person who can't afford the regular price can get a chance to read books. A lot of elderly folks on small fixed incomes don't have any other way of buying books. Also, there are times I've accidentally run out of copies of my own books and have been thankful I can find one or two at the used book store to keep. Ditto historical research books. Can't afford to buy them new, no way, no how, particularly the ones with pictures in them. I would if I could, but many of them are out of print anyway.
Libraries--bless 'em! Books are sold to libraries new, and are never, never stripped and returned to the publishers. When the books wear out, the libraries buy more NEW copies. This is a GOOD thing.
People who sell stripped books (books without covers)--may they suffer a thousand torments. This is an illegal activity. When a book is stripped, the covers are returned to the publisher, and the publisher reimburses the bookseller for the books not sold. The books themselves should be thrown away, or be _given_ away at most. Instead, some nasty people are taking the publishers' (and author's) money saying they weren't sold, AND pocketing the money from sales of those same books, which is fraud. These are people who don't really care about books, they just want the money and are willing to do something illegal to get it. Also, they're heartlessly defacing and mauling books for profit.
But I do understand some authors' concerns. Keep in mind that the average author earns somewhere around $4,000 a year from their books, and yes, that includes the million dollar incomes of big best sellers in that average. Take out the few best sellers out of that average, and you can imagine what other authors earn. Not much. Most authors I know work full time at an outside job, or depend on a spouse to support them if they're so lucky as to be married. So you're looking at a person is probably working 40 hours a week on the job, and working an additional 15 to 20 or more hours per week trying to put out a book every year or two, on top of juggling a home and family. After a while, they get really, really tired and a little bit cranky about lack of sales because if there were more, they'd probably not have to work so many hours.
25 copies not bought...that's not much in a print run of, say, 40,000. But if it were in the hundreds or more, then it gets a little more iffy. I doubt it'd get that high, though. By the time a single paperback book goes through even 100 readers it'd probably be falling apart.
As a result, I tend to think Bookcrossing.com is probably much better advertising for an author than sales-stealing.