From food, clothing, and shelter to automobiles, computers, and pharmaceuticals, the list of human needs is endless. Since man must produce the values he requires to survive, each of these needs opens up an enormous field of work and study. No one man, no matter how great, can master every human endeavor; every minute spent tilling a garden to satisfy one's need for food is one minute not spent knitting sweaters to keep oneself warm in the winter. If every individual had to produce alone everything he needed to survive, the human race would likely have died out long ago. It would be far more efficient if one person could concentrate his efforts on growing food, another on making clothing, and another on building homes. This is the principle of the division of labor.
Other people can be of great value to a man; they can provide him with innumerable benefits, both material and spiritual. The only question is how a man should go about trying to attain these values from others. The most immediately obvious method is to simply take the things one needs. This may provide for one's survival in the short term, but in the long term it actually destroys the ability of others to produce values, and therefore to be of any value to oneself. As an example, say that your neighbor has a vegetable garden which has just come to fruition. He labored in that garden for weeks, and now, after months of waiting, his hard work has finally paid off. You spot a small patch of carrots which look extremely delicious, and seeing that your neighbor isn't home at the moment, you walk over to his garden, pull up the carrots, and take them home with you, enjoying them that night at dinner. Although you have gained a small value from your act (the filling of your stomach for a few hours), you have, in fact, caused a great amount of harm to yourself in the process. When your neighbor arrives home he will be distraught; the primary reason he planted his garden in the first place was to feed his family and enjoy the fruits of his labor. If this benefit is denied to him, what motivation does he have for planting his garden again next year? Furthermore, he may have been counting on selling some of his carrots in order to purchase seeds for the next year's crop. He now has nothing to sell, and so no way of planting his garden again even if he wanted to. Even if your neighbor never finds out that it was you who took his carrots, you have lost a potentially great source of value in the future. If you do this to enough people, you will eventually find yourself without any source of food at all. A much better way to go about acquiring food from your neighbor is to offer him something in exchange for it, perhaps something of which you have an overabundance but of which he has very little. If you make him a good enough offer, he will accept, and you will both end up benefitting from the trade.
The trader principle is a recognition of the fact that the most effective way of gaining values from others is not by deception or theft, but by trading value for value to the mutual benefit of both parties. This is the application of the principle of justice to one's dealings with other men: an individual must not seek from others that which he has not earned, i.e. that which he has not "paid for" to the satisfaction of the other party.