Is the owner of a bookstore sinful if he gives a woman a blank piece of paper and glue, while suspecting that she will use them for witchcraft? And what is the expiation for that?

Rulings are based on certainty or predominant conjecture, not mere doubt. Predominant conjecture is treated as certainty, and rulings are built upon it. If what the questioner has is mere doubt, then there is no blame upon her. However, if she has a predominant conjecture due to circumstantial evidence and the prevalence of magic in her environment, then she erred by neglecting her conjecture, and she must act according to her predominant conjecture. Since she did not intentionally aid someone who wished to practice magic, she must seek forgiveness and rectify the situation if it occurs again.