Is there a sin upon one who buys inexpensive books that he suspects are "pirated" (not original and printed by illicit means), thereby depriving the author of material benefit?

It is not permissible to buy photocopied books without the permission of their owners if the buyer knows this or strongly believes it to be the case, for it involves cooperation in aggression. However, mere doubt does not prevent it. It was reported that Sheikh Ibn Uthaymeen (may Allah have mercy on him) stated that photocopying is permissible for personal use, not for commercial purposes, and that it is not permissible to buy from a seller who sells photocopied books unless he has permission from their owners. He also stated that there is no sin upon one who buys while unaware of the situation.