Even if one performs Bedikat Hametz (the search for Hametz) and declares the Bittul (renunciation of ownership over Hametz), he should still arrange for the sale of all his Hametz to a gentile. Even after the Bedika and Bittul, he might have in his possession items such as vitamins and cosmetics that may contain Hametz; these should be stored away and sold to a gentile.
Rabbi Yosef Shalom Elyashiv (contemporary Halachic authority in Jerusalem) holds that one should preferably specify when selling his Hametz where precisely in his home the Hametz will be situated. One should mention in very specific terms in which rooms and in which pieces of furniture in those rooms he is storing the Hametz. Some people even have the practice of giving a key to the gentile, or at least informing him where the key is kept; since one transfers ownership over the Hametz to the gentile, the gentile must be given access to that Hametz.
According to the Rambam, one need not make a Kinyan (formal act confirming an agreement, such as shaking hands) when authorizing the Rabbi to sell his Hametz. Therefore, strictly speaking, so long as one makes it clear to the Rabbi that he sells his Hametz to the gentile, either in person or by phone or fax, the Rabbi is empowered to sell the Hametz. Many people have the practice to nevertheless perform a Kinyan usually by taking a handkerchief and this is a worthwhile practice, but strictly speaking, the sale is effective even without a Kinyan. (Rav Elyashiv)
Preferably, one should not keep in his home "Hametz Gamur," actual Hametz such as bread and cereals, on Pesach. These items should be removed from one's home, rather than sold. The sale of Hametz is intended mainly for items of questionable status or that consist of Hametz mixed with other ingredients. Actual Hametz, however, should be removed from the home. Nevertheless, if one did sell actual Hametz, the sale is valid and the Hametz is permissible for use after the holiday.
One who lives in a rented residence may sell the property to a gentile for Pesach, and need not ask permission from the landlord.
Owners of factories, supermarkets and stores including drugstores and variety stores must sell their Hametz to a gentile before Pesach. They may not retain ownership over any Hametz in their store during Pesach, as they would then violate the prohibition of Bal Yera'eh U'bal Yimatzei (owning Hametz on Pesach).
Summary:
1) One must sell his Hametz to a gentile even if he searches for Hametz and makes the Bittul declaration.
2) One should preferably specify precisely where in the home the Hametz is situated.
3) Strictly speaking, it suffices to make it clear to the Rabbi that he authorizes him to sell his Hametz to a gentile on his behalf, and no symbolic act is required. Nevertheless, many have the custom to pick up a handkerchief.
4) One should preferably remove from the house before Pesach all actual Hametz items, such as bread, cakes, cereals and the like, rather than sell them to a gentile.
5) One may sell his Hametz even if he lives in a rented residence, without asking the landlord for permission.
6) Storeowners must sell all the Hametz in their stores before Pesach.
