In recent generations, new storage methods have been introduced that allow us to preserve food products for long periods of time. As a result, food manufacturers and dealers are in constant possession of large inventories of food, and they need to sell their ḥametz before Pesaḥ in order not to lose the value of their stock. Moreover, if food manufacturers were to make a point of exhausting their entire inventory before Pesaḥ, it would take days and even weeks to restock and market their products. In addition to the loss it would cause to manufacturers and merchants, it would cause inconvenience to those unable to purchase ḥametz foods during the weeks after Pesaḥ. Therefore, factory owners, food chains, and stores sell all of their ḥametz to a gentile before Pesaḥ, and as soon as Pesaḥ passes, they buy it back again and remarket it.
In principle, anyone may sell his ḥametz to a gentile via the mekhirat ḥametz organized by his local rabbis. He may do so even if he only wishes to sell a small amount of ḥametz – for example, a package of pasta – because once it has been sold, the Jew no longer violates the prohibitions relating to ḥametz.
Some are stringent and prefer not to rely on mekhirat ḥametz since it appears fictitious: the ḥametz remains in the Jew’s house, the gentile will almost certainly not come to take it, and the Jew resumes eating the very same ḥametz as soon as Pesaḥ is over. According to these poskim, it is only proper to sell ḥametz in order to prevent a great loss; concerning a small loss, one should not sell his ḥametz.
Nowadays, all are advised to participate in mekhirat ḥametz, because some food products and flavored medicines may contain small amounts of ḥametz, and they should not be destroyed just because of this possibility. On the other hand, these must not be kept because they may actually contain ḥametz. Therefore, the proper thing is to sell them and thus avoid all doubt. In addition, some maintain that people who have stock in companies that own ḥametz must sell their shares; the shares are sold with the rest of the ḥametz.
Concerning ḥametz gamur, it is customary to advise people not to sell insignificant amounts of ḥametz, so as not to use the sale for minor needs. However, when significant loss is involved, it is permitted le-khatḥila to sell the ḥametz. Each person should decide for himself what he considers a significant or small loss. There is no reason to ask a rabbi about this, and there is no reason to ask what is ḥametz gamur and what is uncertain, as the basic halakha is that even the sale of real ḥametz of little value is permissible, for the sale is valid even le-khatḥila, and one may rely without concerns or questions on the rabbis conducting the sale.
Because the sold ḥametz remains in the Jew’s house, there is a risk that he will forget that it is prohibited and eat it during Pesaḥ. Therefore, one must set up a partition at least ten tefaḥim high (c. 80 cm) to act as a barrier between himself and the ḥametz or lock the ḥametz in a cupboard and hide the key. One may also tape up the cupboard. It is best to write “sold ḥametz” on it so that one does not accidentally open it on Pesaḥ (see SA 440:2).
Bedikat Ḥametz U-vi’uro 8:17; 22, states that in addition to bitul, one should sell all of the ḥametz in his possession, since if he was unaware of a piece of ḥametz during the time of nullification and did not nullify it properly, the ḥametz sale will protect him from violating any prohibition. In my opinion, however, this is not the proper course of action, because such a sale looks like a fiction, and it has no value (as opposed to stocks, which have value). Regarding ḥametz that one forgot about, the Sages instituted bitul, not sale.
. Some rule that the sale should only be done for the great needs of owners of large businesses, but individuals must take care, as much as possible, to only sell uncertain ḥametz, not ḥametz gamur (Bedikat Ḥametz U-vi’uro 8:9; Sidur Pesaḥ Ke-hilkhato 11:16 inclines toward this as well; but see Piskei Teshuvot 448:10). According to the letter of the law, anyone may rely on the sale, since the sale is valid according to the majority of poskim. Even if the seller feels that the sale is somewhat fictitious, if the gentile wishes to do so, he could take the seller to court and legally claim the purchased ḥametz, which proves that the ḥametz indeed belongs to the gentile. Additionally, some say that since every person nullifies his ḥametz, and subsequently there is no Torah prohibition on this ḥametz (there is only a rabbinic injunction that requires destroying or selling the ḥametz in addition to bitul), the ḥametz assumes the status of a safek de-rabanan (an uncertainly about a rabbinic prohibition). Thus, one may certainly rely on the lenient opinions that permit the sale.