As a selfish prizefighter, I want to beat my opponent. If I was an altruist instead, I don't think I'd be able to win one fight. Because I am in fact selfish, fighting an opponent who is an altruist would not do much for my self-esteem. Only in fighting better fighters than I am do I learn, not by fighting someone inferior. If a superior fighter does not do his best in a given match with me for some reason, I cannot objectively pretend to be better than him just because I won once. It benefits me to beat him when he's at his best. I like to share my knowledge, so I teach others. It benefits me when someone learns a technique I teach them well, and puts their own take on it. Thus my student becomes my teacher, and I am that much better off for it. There may come a time when my student defeats me, and though I will probably be upset about getting old and slow, a part of me will be proud of him, and of myself.
Anyone I've met that's worth their salt is generally not afraid of their own shadow, and don't horde ideas or knowledge, afraid that someone will outdo them. Regardless someone always does. If in life you either sink or swim, merely floating is like compromising between life and death, and between the two, only death gains from life not vice-versa.
It's a philosophy of life, so of course there will be people who disagree, or don't really follow even if they do agree. But in terms of what kinds of people gravitate to each other, even if you disagree you're probably more likely to gravitate to people who are good at what they do and are willing to teach you. Thus I have met people who are sufficiently selfish, but not necessarily objective or good at what they do, and a load of other permutations, but I've never met someone who's exceptional at what they do who isn't selfish. You don't get good by not knowing what you want and not achieving it.
Selfishness is not in proportion to salary. Satisfaction and general happiness with personal values and their achievement however, is. If a person is happy to accept a job that pays less but that he himself perceives as a personal value is selfish. Accepting the higher paying job which is not in accordance to your values or against them is like selling your values to the highest bidder. If you allow your non-values to feed you and your kids, contradictions will develop. That would be a sacrifice, because in objective reality, good is that which sustains life, unless life is serfdom to non-values.
The ultimate judge of this is individual perception and judgement of objective reality, whether 99% correct or just 40%. The difference between a selfish person and an altruist is that the selfish person wants to know the difference, which is to say that the selfish person wants, which constitutes a value to gain. An altruist is torn between values. When buying an iPod, the selfish person enjoys his ability to do so from the labor he respects. The altruist whether in a job he likes or dislikes will be emotionally torn between spending money on himself instead of on others.
A person who steals an iPod cannot be said to be selfish, if words are to mean something. If we agree that selfishness paves the way to the achievement of values, which are by definition life-affirming, when a person resorts to violence instead of productive labor, to obtain something he wants by using violence, a non-value, non life-affirming, or to think of it in another way: as a loss of his ability to claim that one cannot use violence against him, he is committing a sacrifice by exchanging what is life-affirming--living by his own achievement--to force, something that could justifiably be used against him to deprive him of that which did come to him by values.
While people are not like clockwork, choices will be made in proportion by the individual's need and ability to know. The deeper the need and the knowledge, the more objective the choice.