[14] Beit Yosef explains that there are two types of prohibitions: It is clear from Tosafot in Megilla 5b that one may not trade in luxury items that bring joy. In Yevamot 43a, however, Tosafot cite an opinion that states that one should curtail all business dealings. The formulation in sa seems to rule in accordance with both opinions, as they do not contradict each other. sa 551:2 deals mainly with transactions that bring joy, whereas sa 554:22 implies that one should curtail all business dealings. mb 551:11 states that it is customary to be lenient regarding regular transactions, because it is otherwise difficult to make a living. This is why I wrote that the main prohibition relates to items that bring joy and added that one should, ideally, curtail all business dealings. See Torat Ha-mo’adim 5:16, which states that regular transactions are completely permitted, even defining cabinets and appliances as items that do not bring joy. However, Igrot Moshe, oĥ 3:82 states that purchasing bookcases brings one joy, and one should, therefore, avoid doing so during the Nine Days. It seems that one should refrain from buying anything that brings joy, unless there is concern of financial loss. Under such circumstances, the Sages permitted engaging in transactions, just as they permitted building a wall that brings joy, if there is concern that not doing so will result in damage (mb 551:13). This is why I wrote that merchants may continue their business dealings as usual, even if they deal in luxury items, because they are liable to sustain a loss if they stop working. Several Aĥaronim follow this position as well; see sht ad loc. 13. (These merchants are not helping others commit a transgression, because it may be that their customers are buying for the sake of a mitzva and cannot postpone their purchase. And even for those who are violating a prohibition by buying, since they can make their purchase elsewhere, some say that the prohibition of lifnei iver – putting a “stumbling block in front of the blind” [Vayikra 19:14] – does not apply to rabbinic commandments.)

If postponing the purchase of furniture and clothing for a wedding would cause the wedding to be delayed, one may purchase these items during the Nine Days, because doing so is for the sake of the mitzva of marriage. This is allowed only if the groom has not yet fulfilled the mitzva of procreation (Rema 551:2, mb 551:14). Nowadays, however, postponing these purchases does not delay the wedding, because people book a wedding hall and send out invitations months in advance, and it is possible to buy everything before or after the Nine Days. Either way, no one would cancel a wedding on account of these purchases. Only in rare cases, when the families are under great pressure, may they buy clothing, as poskim typically permit many things in situations of great need. I have written that, in practice, one must recite She-heĥeyanu over the purchase of Torah books, which bring people great joy (Peninei Halakha: Berakhot 17:9). Nonetheless, one may buy Torah books during the Three Weeks. One should refrain from arranging these books on one’s shelves upon buying them until Shabbat, at which point one should begin learning from them and should then recite She-heĥeyanu (above, section 8). Then one should arrange the books on the shelves on Motza’ei Shabbat.