Since it is prohibited to possess chametz on Passover, any chametz left undisposed must be sold to a non-Jew.

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All such chametz, as well as all chametz utensils that were not thoroughly cleaned, should be stored away. The storage area should be locked or taped shut for the duration of the holiday.

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Since there are many legal intricacies involved in this sale, a rabbi acts as your agent both to sell the chametz to the non-Jew on the morning before Passover, and also to buy it back the evening after Passover ends.