Selling books of the previous class after being promoted to higher class to a second hand bookshop is not a business activity.
The student has been promoted to a new class and now the books of the previous class are useless for him. So he intelligently sells the books to a second hand bookshop, so that they could be used by other students.
This is not a business activity because this is not done on a regular basis by the student. It can hardly be done once in a year after being promoted to new class. So it shouldn't be taken as a business activity.