In general, one is forbidden from performing melacha on Chol HaMoed. The broadest leniency is that if someone will lose money or something else that they already have (davar ha’avud), then they can do melacha even if that requires “skilled labor” (meleches uman). For example, if a freezer breaks it can be repaired to avoid the loss of the meat inside it, or if a Mashgiach notices a kashrus issue he do whatever is required to prevent people from eating non-kosher. But this does not justify melacha done to earn a profit or gain something that they do not already have, and therefore one cannot buy things even if they are on sale.
One may also perform melacha on Chol HaMoed if that provides benefit or comfort on Chol HaMoed or on the latter days of Yom Tov (tzorech HaMoed), but in this case one may only do melacha that does not require skilled labor. For example, one can cook, check vegetables, wash dishes, buy supplies, or drive a car in order to service restaurant customers on Chol HaMoed or take their family on a Chol HaMoed outing. But one cannot cook or place orders for after Yom Tov, paint their house, or perform general maintenance.
A person may build or expand a sukkah on Chol HaMoed, repair a sukkah that fell down or was damaged, and buy or build any of the “parts” needed for a sukkah.
Laundering clothing and other items is generally forbidden even if it is done for use on Chol HaMoed or Yom Tov. An exception is that items which became soiled on Yom Tov or Chol HaMoed can be washed, and the same applies to towels and the clothing of very young children.
A person can write down information on Chol HaMoed to avoid forgetting it. For example, a waiter may write down a customer’s order, a person can write his shopping list or a Torah thought that he might forget. Writing which is merely done to help add to one’s enjoyment of Chol HaMoed (a check to pay for entertainment) should preferably be written in a somewhat unusual manner (shinui).