Shopping is forbidden on Yom Tov, so there's also presumably a prohibition on it on Chol Hamoed. What are the parameters within which it's permitted on Chol Hamoed?
In The Laws of Pesach (2005), by Rabbi Avrohom Blumenkrantz, it says:
- The Sages prohibited commerce on Chol Hamoed even when it entails only buying and selling and no actual melacha. Commerce was forbidden since it often involves physical exertion or mental stress which detracts from the festive atmosphere of the day. However, even small, simple transactions which do not involve exertion were also included in this prohibition.
Commerce is prohibited whether one personally buys or sells, or uses a non-Jew as his agent. It is prohibited whether one buys or sells for his personal needs or to earn profit for a business. However, just as melacha is permitted on Chol Hamoed in certain intances, so too is commerce permitted in these same circumstances. Thus, one may buy or sell when necessary for the festival, to prevent a loss, or to provide a poor merchant, who lacks food, with the basic necessities. In addition to these general exemptions, commerce, since it entails only exertion but no melacha, is permitted in other instances as well.
26.One may not buy stocks or commodities on Chol Hamoed since it is unlikely that one of the aforementioned exemptions would apply to such purchases. However, one may sell a stock on Chol Hamoed if there is a reasonable possibility that he might otherwise incur a loss, or if he needs the money for Simchas Yom Tov.
-Page 13-464 (emphasis mine)
So we have a few things:
when necessary for the festival
to prevent a loss
or to provide a poor person with the basics