A person should ensure to sell his Hames to a gentile before Pesah even though he thoroughly searches his home, eliminates all Hames that he finds, and makes the Bittul declaration renouncing ownership over any Hames. The sale is necessary in case he owns some medications or cosmetics that contain Hames, or in case there is some other Hames in his possession of which he is unaware. The Sages enacted a provision forbidding deriving benefit from Hames that was owned by a Jew during Pesah. Even after Pesah, it is forbidden to derive any benefit from Hames that a Jew had owned during the holiday. This is a penalty imposed by the Sages upon those who keep Hames in their possession over Pesah. Therefore, if a supermarket is owned by a Jew and he does not sell the Hames in the store to a gentile before Pesah, it is forbidden to purchase from that store any of the Hames that it had owned during Pesah. One must therefore determine which stores are Jewish-owned, and which Jewish owners sell the Hames to a gentile before Pesah. Rav Yosef Shalom Elyashiv (contemporary) addresses the situation of a non-observant Jew who owns a supermarket and formally signs the sale document only so that he could retain his kosher certification. Rav Elyashiv rules (listen to audio recording for precise citation) that since this owner does not sincerely intend to sell his Hames to a gentile, but rather simply goes through the motions, the sale is not valid. Therefore, it is forbidden to purchase Hames from such a store after Pesah. Rav Elyashiv adds that it is forbidden to buy Hames from this store even to feed young children. He advises in such a situation that a clause be added to the sale document stating that the sale is done in accordance with the country's laws. Once the sale is binding according to civil law, the storekeeper clearly intends to transfer ownership over the Hames, and hence the Mechirat Hames is valid. It is preferable when selling the Hames to either tell the gentile or write in the document where the key to the premises is located. Since the Hames becomes owned by the gentile, he should have access to it, which requires access to the key. When one sells his Hames to a gentile through the agency of a Rabbi, as is the customary practice, it is not necessary to make a Kinyan (formal act of affirmation) with the Rabbi. It suffices to verbally or in writing authorize the Rabbi to sell one's Hames on his behalf. This can be done via telephone, mail, e-mail, and so on. This Halacha is based upon the ruling of the Rambam (Rabbi Moshe Maimonides, Spain-Egypt, 1135-1204), who held that a Kinyan is not necessary when appointing somebody as one's agent. One does not have to sell the particles of Hames in the walls of his pots and other utensils. This Hames does not fall under the prohibition against owning Hames, and therefore it does not have to be included in the sale of one's Hames. Mechirat Hames is done by selling to the gentile the space where the Hames is located. The sale document states that the gentile assumes ownership over the area where the Hames is situated, and then by extension he acquires the Hames, as well. In light of this, we might, at first glance, question whether one can sell his Hames to a gentile if he lives in a rented residence. Since he is not the owner, he seemingly does not have the legal power to sell any part of the residence. In truth, however, as Rav Elyashiv writes, it is legitimate to sell one's Hames to a gentile even if he lives in a rented residence. Since it is obvious that the gentile will not come and move in to the property, it can be assumed that the landlord does not object to the sale, and allows the tenant to proceed with the Mechirat Hames. It often happens that one will want access during Pesah to the room where the sold Hames is stored. In such a case, it is proper to include in the sale document a stipulation to this effect. Since the gentile becomes the owner over the area where the Hames is kept, it should be made clear from the outset that the seller will continue to enjoy the right to access to the room in question. Mechirat Hames affects only the Hames that one has in his possession at the time when the sale takes effect. If Hames comes into his possession at some point during Pesah, it is not included in the sale, and he is therefore in violation of the prohibition against owning Hames on Pesah. Some people order bread from non-Jewish bakeries before the final days of Pesah, and the bakeries deliver the products on the last day of Pesah. If this happens, he must not accept the delivery, because if he does, he violates the prohibition of Hames. Since that bread was not sold to a gentile, if it comes into his possession he will be in violation of this prohibition. One should therefore ensure that no Hames will be delivered until after Pesah. Rav Moshe Sternbuch (contemporary) records a custom to pay the Rabbi a small amount of money when one comes to him for Mechirat Hames. The reason for this custom is that if the Rabbi is paid for the service, he will see himself as the individual's "employee," hired to sell his Hames on his behalf. This will ensure that the Rabbi will follow through on this responsibility and properly execute the sale as required.