Merchants who followed the armies and sold the various clothing and food items at extremely high prices were called?

Profiteers, but there may be other more specific terms. Profiteering can exist in peacetime as well as war, when greedy merchants see the opportunity to sell necessary goods at exorbitant prices, knowing that their customers have no choice but to pay it. During the American Civil War merchants were licensed to follow the US Army and sell goods. They were called sutlers; some of whom were honest merchants, and others of whom engaged in ruthless profiteering.