Capitalism in its purest form, like socialism in its purest form, do not take into account human nature. They both operate well on paper and in the ether of "theory".
However, when human nature is added to the equation capitalism becomes a form of darwinism. Only the strong (or those with money) will survive. Is it just that the one person in line who had the money to pay $15.39 for the ice was able to buy it whether he had a greater need or not?
Capitalists forget that we live in a society that, to insure its success, must take into account the diversities of human nature. Some people will have more than others and those that can't provide for themselves will need some help.
Capitalists refuse to see this. For them they only exist "to maximize profits".
Sure it does.
The guy who didn't charge the absolute top dollar "then and there", because he knew that he'd pay dearly for it later on, was indeed taking human nature into account, and maximizing his profit.
Maximizing long term profit is not anathema to property rights or capitalism.
By the same token, I rather suspect that his suppliers (commodities such as fuel, food, etc.) are charging him what the market will bear, and not being "mister nice guys" for his benefit.
Different strokes for different folks. Different markets take different marketting.
Local merchant doing local business on a long term basis, yeah, he's gonna have to market differently than a petroleum processor or a drug manufacturer.
But ultimately, the merchant that doesn't do what's in his own long-term best interest is not going to be around to have a "long-term" as a merchant, because someone else will come into his territory and cut him off at the knees.