The answer is price. The price of a good conveys about its relative scarcity or abundancy. If the price is high, the good is scare meaning you can gain money by selling extra of it, and you can save money by buying a lesser amount of it. If you act according to your self-interest, selling more and buying less of that costly good, the scarcity of that good will be toned-down. If the price of a good is low, you can exhilarated to do the contrary, thus removing any excess of the good in the market.