Is it permissible to sell non-religious books and use their price to buy religious books or for charitable work and sadaqa?

The basic principle regarding sales is permissibility, as Allah Almighty says: (And Allah has permitted trade and forbidden usury). However, if the item being sold is forbidden, or is predominantly used for forbidden purposes, then it is not permissible to trade it.

Books are either: 1. Beneficial and useful (even if not in religious sciences) and do not contain forbidden content: In this case, it is permissible to sell them and use their price. 2. Forbidden in themselves or harmful to the reader, such as those that contain lies or corrupt morals and religion: In this case, it is forbidden to sell them, and they must be destroyed. It is not permissible to benefit from their price, as the Prophet, peace and blessings be upon him, said: (Indeed, when Allah forbids something to a people, He also forbids its price).