Markets

This is the consensual exchange of goods/services between parties. It uses the price system to ascribe a subjective number value that represents the cost to attain those goods/services.
This price being subjective, mellows out according to what market participants are willing to pay for said goods/services at a particular time which is how we arrive at the market price.

This is also a positive sum game since both traders are receiving something in greater subjective value than they are losing.

Jim has $20 and sees that a seller has apples for $5 per apple
Jim buys an apple, meaning he values the apple over his $5
The seller conversely values $5 more than an apple