We are taught to be careful with חמץ שעבר עליו הפסח. This refers to unsold Chametz that was left in the possession of a Jew during Pesach.
We are supposed to ask store owners to produce a certificate that says, אישור מכירת חמץ, proving that the Chametz was sold and that place did not violate this prohibition.
Most are not aware that חמץ שעבר עליו הפסח is a rabbinic enactment. If you think about it, this has to be rabbinic. For if someone ate unsold Chametz, he is still eating Kosher food, and it’s no longer Pesach. Nevertheless, because eating Chametz on Pesach carries with it a כרת punishment, we are meant to be very careful to handle our Chametz with care.