Chametz for Sale

There is a well-known and widely used technique to free oneself from violating the prohibition of owning chametz on Pesach — selling one’s chametz to a gentile.
Famously, the Vilna Gaon was against this practice. However, mechiras chametz has found widespread acceptance in the general community. Still, there are some individuals who follow the Vilna Gaon and do not sell their chametz.
On the other hand, Rav Shlomo Zalman, zt’l, said that even if one does not own any chametz, he should still arrange a mechirah. This is a hiddur mitzvah to ensure that even minute amounts of chametz will not be owned by the pious individual over Pesach.
Since owning chametz on Pesach involves a Torah prohibition, some individuals do not sell bona fide chametz, such as Cheerios and noodles. However, these individuals may purchase chametz from a Jewish store that relied on such a mechirah. Even if the sale for whatever reason was not valid, on a Biblical level one may eat chametz after Pesach that was owned by a Jew on Pesach, r’l. However, there is a rabbinic restriction against doing so. Therefore, even if one is stringent not to rely on mechiras chametz to evade a Torah prohibition, he may rely on the mechirah to avoid violating the rabbinic prohibition of chametz she’avar alav ha’Pesach, consuming chametz after Pesach that was owned by a Jew during Pesach.