Must a consumer purchase some quantity of each commodity to be in equilibrium?

No, a consumer does not need to purchase some quantity of each commodity to be in equilibrium. Consumer equilibrium occurs when a consumer maximizes their utility given their budget constraint, which can happen by consuming only one good or a combination of goods. The key is that the marginal utility per dollar spent on each good is equal, leading to an optimal allocation of resources. Thus, it is possible for a consumer to achieve equilibrium with a preference for only certain goods.