Prices are simply ratios of one thing for another.  Goods and services are bought and sold with money prices outside of the occasional barter.  Prices are the agreed upon ratios at which the parties exchange.  Both parties expect to receive more than they give away.  The exchange will not happen if one of the parties expects to receive less than he gives away.  The recurrence of these ratios sets the expectations for future prices.

The exchange always has two sides.  There’s a seller and a buyer.  One party will view it as a sale.  The other will view it as a purchase.  As this becomes common, people produce, not to consume, but to sell.  The seller then acquires money that can be used to purchase something else.  The division of labor and indirect exchange make it possible so you don’t have to produce everything you need.  You can acquire it indirectly.

It’s assumed that every person has perfect knowledge.  This is not true.  Some have more information and have better foresight.  This gives us profits and losses.  Profit and loss wouldn’t exist with perfect knowledge.  Changes will happen after the exchange.  Some can be aware of the new data, but it’s impossible that someone can be always aware of all data.  Those who have more data and are more farsighted are usually the successful investors and entrepreneurs.

There are always profits to be earned so long as we are not superhuman.  We are never in an equilibrium, nor can it be achieved.  An entrepreneur can see a demand for a product before anyone else.  The entrepreneur will then profit from this.  This will encourage others to enter the market.  Profits will get smaller and smaller until they disappear.  Losses also encourage entrepreneurs to leave a particular market.

Prices are just ratios.  Crusoe can’t exchange his fish for chicken.  He must first exchange his fish for berries.  Not for consumption, but to exchange the berries for chicken.  Crusoe sells his fish to Friday for berries.  Then, he uses those berries to buy chicken.  Crusoe is producing fish to buy other goods.  This is an indirect exchange.  He can use the berries to purchase chicken days later.  He can’t do that with fish. 

Reference

Ludwig von Mises; Human Action