Halacha imposes certain limitations on making purchases during Hol Ha’mo’ed. While it is clear that one is allowed to purchase food and other items that he needs for the holiday, one may not purchase extra items for after the holiday. Meaning, if a person is shopping for the holiday and figures that he might as well buy large quantities to last him even beyond the Yom Tob, this is forbidden. Rav Moshe Feinstein (1895-1986) clarifies that one does not have to make a precise calculation of his needs for the Yom Tob, but it is forbidden to knowingly purchase products that will not be needed until after the holiday.
The Halachic authorities address the question of whether one may buy during Hol Ha’mo’ed products which are sold in bulk. For example, if a person is shopping for the holiday in Costco, and the item he needs for the Yom Tob is sold only in a quantity that is far larger than what he needs during the holiday, may he purchase the product? The Halachic authorities rule that since this is how the store sells the product in question, and generally the price is discounted when buying in bulk, one may make such a purchase. Thus, for example, if one needs a couple of towels for the Yom Tob and the only towels in the store are sold in units of 12, he may purchase a unit of 12 towels.
Rav Moshe rules that one may not return an item to a store for a refund during Hol Ha’mo’ed. As this is a transaction which is not needed for the Yom Tob, it is forbidden. Rav Moshe makes an exception in the case where one needs money for his Yom Tob purchases. If one has no money and needs cash to be able to buy what he needs for the holiday, then he may return items to a store to get his refund. Another exception is a case where the refund is only granted for a limited time. If one will lose the refund if he does not return the item until after Yom Tob, then he may return it during Hol Ha’mo’ed in order to avoid this financial loss.
Finally, Rav Moshe ruled that one may not buy or sell shares of stock during Hol Ha’mo’ed, as these transactions are not necessary for the holiday. Here, too, an exception is made in the case of somebody who needs money for his Yom Tob purchases; such a person may sell shares of stock so he can buy what he needs for the holiday. Another exception is one who knows that if he does not sell his stock, the stock’s value will go down below the principal price at which he purchased it. In such a case, one may sell the stock to avoid a financial loss. However, one may not sell the stock in order to avoid losing profit, as transactions on Hol Ha’mo’ed are allowed only to prevent financial loss, and not to avoid loss of profit.
Summary: It is permissible to buy on Hol Ha’mo’ed items which one needs for the holiday. Other purchases may not be made, except to avoid a financial loss. One may buy items in bulk if this is how they are sold, even if he does not need the entire quantity for the Yom Tob. One may sell something or return merchandise for a refund on Hol Ha’mo’ed only if he needs the money for his Yom Tob expenses, or to avoid a financial loss. Selling to avoid a loss of profit is not allowed on Hol Ha’mo’ed.