Law students are aware that some of the casebooks they so carefully study come in a "Teacher's Edition." That edition is not for sale to students. The teacher's editions of casebooks run the gamut from perfunctory to works of art. The Holy Grail of Teacher's editions is the one for the Dukeminier & Krier property casebook; it is a book of its own: not only does it have the answers to all of the questions posed, it has explanations of them, as well as historical material, everything a neophyte professor needs to come across as an expert, and everything a student would need to ace the course.
Many other fields of education have teacher's editions too, and the same inhibitions on them falling into students' hands apply. That's where eBay comes into play. Just as book reviewers have historically sold to second hand bookstores copies sent by publishers, and disk jockeys the albums they are given, some professors have taken to selling the teacher's editions they receive. eBay now provides a vast market of eager students and perhaps others. Educational publishers don't like such sales, and are reported to be trying to prevent sales on eBay of them.
There are no special rules for the sale of teacher's editions. To begin with, the first sale doctrine doesn't require a sale; other countries more accurately describe it as the "exhaustion" doctrine: once the copyright owner has parted with title to the physical object, the copyright owner's rights over that copy are "exhausted" and anyone who lawfully possesses that object can do what they want with it in terms of distribution and display. So, if an educational publisher gives a teacher a teacher's edition rather than selling it, that fact doesn't matter. The teacher can sell it on eBay or wherever.
But what if the copy says, "Not for sale"? Many copies sent to book reviewers and to disk jockeys contain similar legends and can be found in second hand stores; I own a few myself that I have purchased. One would have to ask whether there is a contract under such circumstances to determine whether there is a breach of it, since the first sale doctrine does not apply to licensed only uses. There are reasons to be skeptical there is a license only in such a situation (it is, for example, highly unlikely the publisher has an expectation of getting the copy back in the event of a breach), but if there is a contract, it is only between the book publisher and the original teacher, not a reseller.