To be a good Kosher Jew, takes some knowledge. There are several prohibitions surrounding chametz (“leaven”) on Pesach. In addition to the prohibitions of eating and benefiting from chametz during Pesach, there are two Biblical prohibitions which one violates merely by possessing chametz over Passover: bal yira’eh and bal yimatzei—chametz shall not be seen nor found in one’s possession during Pesach (based on Shemot 12:19 and 13:7).
There is also a positive commandment to dispose of one’s chametz on Passover eve (based on Shemot 12:15). Finally, post-Pesach there is a rabbinic prohibition against benefiting from chametz that was owned by a Jew during Pesach.
In order to comply with these injunctions, two methods of disposing of chametz are traditionally employed. The method used throughout much of history (when most people did not have pantries laden with food) was simply to destroy all of one’s chametz, preferably by burning. Out of the concern that one may not be aware of all extant chametz, The Rabbis (Chazel) instituted a search (bedikah) before destroying (biur) any found remnants of chametz.
A second method used is bitul, a technical nullification of the chametz by which one declares his chametz to be ownerless and like the dust of the earth. In theory, either of these methods—biur or bitul—would suffice to avoid the Biblical prohibitions; in practice, both are used (Magen Avraham 431:2). Whatever method(s) is used, it must be carried out before the fifth halachic hour on Passover eve (Pesachim 21a; Shulchan Aruch, OC 434:2).
It would seem that an equally valid solution is to give or sell the chametz to a non-Jew. That is exactly what Rabbi Yochanan of Chakukaah advised to do with someone else’s chametz for which he was responsible (Pesachim 13a). That Talmudic story involves a standard, irrevocable sale in which a non-Jew pays full market value for the chametz, takes it home and uses it.
A typical mechirat chametz today differs in that the non-Jewish buyer gives only a small down payment, leaves the chametz in the Jewish individual’s house and after Pesach ownership is transferred back to the original owner.
The earliest source for such a transaction is the Tosefta (Pesachim 2:6), which records that a Jew on a boat may sell or give his chametz to a non-Jewish shipmate and buy it back after Pesach. This is codified by Rambam (Hilchot Chametz U’Matzah 4:6) and the Shulchan Aruch (OC 448:3).
What must be sold? Clearly, there is no need to sell kitniyot (legumes), whose consumption is only forbidden by Ashkenaz custom. Only Chametz itself, plus maybe the utensils that cooked the Chametz.
The utensils themselves present more of a challenge. The question of what to do with chametzdik, non-kasherable dishes is discussed in the gemara (Pesachim 30a). Rav rules that all chametzdik utensils must be destroyed and may not be used after Pesach.
Shmuel disagrees and maintains that they may be used after Passover. The Halakhah follows Shmuel (thank G-d for that), and the Shulchan Aruch states (OC 451:1) that there is no need to sell or otherwise dispose of one’s chametzdik utensils. They simply need to be scrubbed clean of any visible chametz and locked away.
After Pesach they may be used. The common practice is thus not to sell dishes. Such dishes, however, may not be used for food preparation on Pesach—not even for cold food (Rema, OC 451:1). They may be used for non-food purposes (Rema, OC 450:7) and sold to a non-Jew on Pesach (Shoneh Halachot 450:12). The discussion above pertains to chametzdik dishes; vessels that do not contain any absorbed chametz but are merely being used to store chametz are often sold in the contract used for mechirat chametz, similar to the way warehouses that store chametz are sold.
The sale of chametz must be fully binding under Jewish law, and some authorities require that it meet local legal standards as well. It is a complex and technical transaction involving intricacies of Jewish commercial law in which an error can lead to the violation of two Biblical prohibitions. Thus, the custom has developed to have a communal sale administered by a competent and experienced rabbi. The way it is performed today, the rabbi serves as an agent to sell the chametz, but at no point does the rabbi own any of the chametz that he is selling on behalf of others..