סימן קו סעיף א
כָּל סְחוֹרָה, אֲסוּרָה, בֵּין לִקְנוֹת בֵּין לִמְכּוֹר. רַק אִם נִזְדַּמֵּן לוֹ רֶוַח מְרֻבֶּה יָכוֹל לִקְנֹת וְלִמְכֹּר בְּצִנְעָא, וְיוֹצִיא לִכְבוֹד יוֹם טוֹב יוֹתֵר מִמַּה שֶּׁהָיָה בְּדַעְתּוֹ לְהוֹצִיא.
All commerce is forbidden,1The Sages prohibited commerce on Chol Hamoed, since it often involves physical or mental stress which detracts from the festival. (Magein Avraham 539:1, Mishnah Berurah) It is also prohibited because of the fear that you might write something that is not permitted to be written on Chol Hamoed. (see Yeshuos Ya’akov) whether buying or selling.2Shulchan Aruch 539:1. However if you have an opportunity to make a big profit,3In this case, even if the opportunity may be there afterwards, it is permitted, for if the opportunity will be lost, it would be permitted even without the conditions listed. See paragraph 3. you may buy and sell in private.4Ramah 539:1. This restriction is only in a situation when you have sufficient money, or, according to the Taz, when you are extremely wealthy. But if you are not, then you may buy or sell, even openly and for a modest profit; provided you use some of this profit for enjoying the Yom Tov. You also should keep your store gate closed until a customer arrives. (see Mishnah Berurah 539:5, 6, 17, 18, Sha’ar Hatziyun 539:7, Biur Halachah 539:4, and Chayei Adam 112:2) You should then spend more in honor of Yom Tov than you had previously intended.5Ramah. See footnote 4. (Biur Halachah 539:5) You are not permitted to do a melachah (like cutting material to sell) even under these conditions.
תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)