106:3 If, on chol hamoed, a fair comes – meaning a market day that happens only rarely – or if the regular market day comes every week but this time is before yom tov and is expected to attract a larger-than-average crowd, one may do business. This is because these circumstances aren’t common so a loss of profits is considered like a loss. However, one may not do business on a normal market day during chol hamoed. When merchants or ships come infrequently and they are selling cheaply or buying at a good price, it is likewise permitted to buy from or sell to them.
106:4 If one who needs to buy wine at harvest time for his family to drink during the whole year, and afterwards the wine will cost more, then one may buy it on chol hamoed but he may not buy it for doing business. (Some authorities disagree that one may buy wine for the year on chol hamoed. In deference to their position, it is advisable to conduct such transactions in private – Bi’ur Halacha 539:5 s.v. u’mutar liknos.)