People who adhere to the custom of not eating kitniyot may keep them in the house during Pesaḥ and derive benefit from them, for example, by feeding them to animals (Rema 453:1).
One who does not eat kitniyot on Pesaḥ may cook them for somebody who does, but it is recommended that he use some sort of reminder that he is not cooking for himself. It is likewise permissible for a storeowner to sell kitniyot during Pesaḥ; however, if there might be wheat kernels among the kitniyot that make up more than one sixtieth of the mixture, the storeowner may not sell the kitniyot, because this could cause customers to transgress the ḥametz prohibition. It would be better to sell such kitniyot along with the ḥametz.
If kitniyot fall into a cooked food, they should be removed, and whatever cannot be removed is batel in the majority of the dish. If, however, such a large amount of kitniyot falls in that they become the majority, the dish is considered a kitniyot dish and its consumption is forbidden (Rema 453:1; MB 8-9 ad loc.).
It is permissible for one who does not eat kitniyot on Pesaḥ to eat from and cook with clean kelim in which kitniyot were previously cooked.
Those who abstain from kitniyot may still eat food cooked in a pot in which kitniyot had been cooked and that has been cleaned well, since kitniyot are not considered entirely forbidden.
Some maintain that the prohibition of kitniyot begins at the onset of Pesaḥ (Sidur Pesaḥ Ke-hilkhato 16:10 n. 42), and under pressing circumstances one may rely on this view. In practice, however, the mainstream opinion is that the custom of kitniyot corresponds to the prohibition of ḥametz. Thus, kitniyot are forbidden from the time that ḥametz is prohibited (Ḥok Yaakov 471:2; Responsa Maharsham 1:183; Shevet Ha-Levi 3:31).
When the seventh day of Pesaḥ is a Friday, it is permissible (for those who are not keeping an eighth day) to eat kitniyot on Shabbat, though the common practice is not to prepare them on Pesaḥ. Nevertheless, it is permissible to accept kitniyot from someone whose custom is to eat kitniyot on Pesaḥ, and if someone wishes to prepare kitniyot, it is not forbidden.