They may be able to justify the act of purchase, but they won't be able to justify (or usually, even comprehend) how their purchase affects the prices and supply of items on the market. Yet their purchase does exactly that, and does so much better than some central authority setting prices and deciding how much of an item is to be sold. In fact, that's the best system we've found so far of running a market and it depends on millions of people who are only acting for their own selfish reasons and have no idea how what they are doing affects the larger picture.