As you may know, academics receive throughout the year unsolicited copies newly published textbooks from academic publishers. The books are sent for review for possible adoption in courses. The hope is that upon receiving a free book, one will look it over and assign it to one's students. A few adoptions of a given textbook apparently offsets the cost to the publisher of giving a few thousand copies away.
Around this time each year, an elderly couple appears, unannounced, at my office door. They ask if I have any textbooks that I'd like to sell. The couple apparently lives in a mobile home, traveling the country, making money in the used textbook market. I usually sell whatever texts I've received that I've had the time to look at. Since I nearly never use textbooks in my classes (I use primary sources and journal articles), it doesn't much matter what I think of the texts I am sent anyway.
The question is whether it's ethical to sell complimentary review copies of textbooks. Some thoughts: Copies sold in the used market do not yield revenue for the publisher, and so no royalties are paid to the author or editor of the book. On the other hand, the copies are unsolicited, and take up space in my office. And used books save students money. But, again, textbooks are so expensive precisely because publishers need to offset losses due to the used market. Yet I like the elderly couple that shows up at my office, and would like to seem them succeed in their little business.