Business is prohibited on Ḥol Ha-mo’ed. Buying, selling, renting, and hiring are included in this prohibition. For the festival was given to the Jews to enable us to eat, drink, and study Torah. Business is generally burdensome and worrisome, and also has the potential to be upsetting, as when a deal does not go through. True, one minor deal is not so burdensome and worrisome. Nevertheless, since business is open-ended, and something small sometimes morphs into something big and complex, the Sages prohibited all business dealings on Ḥol Ha-mo’ed (MK 10b; Rosh; SA 539:1; MB ad loc. 2; AHS ad loc. 3-4).
Nevertheless, in order to feed people on the festival, one may buy and sell (MK 13a-b). Even if he could have bought all necessary food before the festival, he may buy as much food as he needs on the festival, as the Sages did not want to create limitations likely to reduce festival joy. Food purchases do not need to be minimized. A person may buy generously so that the food will last him for the whole festival, up to and including the Shabbat immediately following the concluding Yom Tov. If any food is left for afterward, he may enjoy it then. However, he may not intentionally buy extra for post-festival consumption (SA and Rema 539:11; see section 3 above).
If people generally buy food in large packages to save money, they may do so on Ḥol Ha-mo’ed as well, since this is how they usually shop. Similarly, when there is a significant discount for buying in bulk, extra food may be bought even if some of it will not be used until after the festival, as this can be considered preventing a loss. However, as with all such cases, the leniency may be taken advantage of only by those who did not intentionally delay until Ḥol Ha-mo’ed, meaning they did not plan to buy in bulk on Ḥol Ha-mo’ed. If one did plan things this way, he may not buy extra on Ḥol Ha-mo’ed, but only what is necessary for the festival (SA 539:1; MB ad loc. 4; see SHT 537:49; compare 12:3 below).
Fundamentally, even non-food items – such as clothes, shoes, kitchenware, electronics, and school books – may be bought on Ḥol Ha-mo’ed if in fact they will be used during the festival. For example, if a woman’s Yom Tov outfit tore or got dirty, she may buy a new dress to wear on Yom Tov. (If she already has a Yom Tov outfit, but would prefer a nicer one, she may not buy another outfit.) In practice, for a number of reasons, this permit is almost never relevant. First, because these are not food items, the permissibility of buying them on Ḥol Ha-mo’ed is conditioned on one’s having been unaware before the festival that he would need them. However, if he knew but did not bother to buy them beforehand, he may not buy them on Ḥol Ha-mo’ed, as he purposefully deferred the melakha until then (MB 539:4 and 540:9; R. Shlomo Zalman Auerbach cited in SSK ch. 67 n. 130).
Additionally, it is prohibited to buy in a store which is not supposed to be open on Ḥol Ha-mo’ed, to avoid supporting transgressors. In practice, almost all stores in Israel which are open to the public on Ḥol Ha-mo’ed are not allowed to be (12:6 below). So the permissibility of buying is limited to buying from one who closes his store on Ḥol Ha-mo’ed but sells privately to clients who make requests, or to buying in a store owned by a non-Jew. One must also be aware that the permissibility of buying clothing or furniture in a non-Jew’s store is on condition that it does not need any professional tailoring or repair. If it does, buying it is prohibited (section 18 below). Note that it is sometimes permitted to buy an item that is not necessary for the festival in order to prevent a loss, as we will explain below (12:7).