Is the buyer sinful if he buys pirated books unknowingly, and is he obligated to inquire about their source before purchasing, given that the original books are unavailable?

There is no blame on someone who does not know that a copy is pirated or printed without the permission of its owners, unlike someone who knows and then buys it, for he is a participant in the transgression. Some scholars permit this for personal use for those who need it and are unable to purchase the original copy.