When we buy new clothes not to keep ourselves warm but to look "well-dressed" we are not providing for any impor- tant need. We would not be sacrificing anything significant if we were to continue to wear our old clothes, and give the money to famine relief. By doing so, we would be preventing another person from starv- ing. It follows from what I have said earlier that we ought to give money away, rather than spend it on clothes which we do not need to keep us warm.
I think the author does not take into account human behavior when making this argument, and he basically defines human necessities as food, shelter, water. He neglects to consider how gaining power and resources to increase fitness is woven into our genetic makeup. Social dominance is a primary characteristic of how humans function in societies, and quality of clothes are often an indication of high social status. I think that the author has faith in human selflessness, but I believe that inherent selfishness will prevent the idea of everyone living at a point of marginal utility from ever becoming a reality. Singer assumes that the majority of the human population has attained an elevated level of moral reasoning that, to be frank, very few people reach. I think that in general, people act in ways that are beneficial or mutually beneficial in terms of elevating status (social status, wealth, etc). I think it is difficult for people to discern a direct material benefit from helping others that may never be able to return the favor. Because of this, we are less likely to help.