Are academic book buyers legitimate?
I agree with Yves that they are selling the books back to students.  Then I think the economics works out as follows:

The faculty selling the books and the book buyers benefit a lot.  The publishers stand to lose some money, since reselling the books hurts the market for new ones.  They presumably pass some or all of this cost on to students through price increases, so the students also lose money in aggregate.  Now the question becomes what price the books are being resold at.  If they are resold cheaply (used book prices), then the students who buy the resold books benefit (they get near-new copies at cheap prices), at the expense of the other students.  If the books are resold at new prices, then no students benefit.