Economics is the study of the distribution of resources. The vast majority of resources are scarce, and have opportunity costs. The few that do not, are called free goods. A few examples of free goods are oxygen, and sunlight. Both provide utility (wellbeing) and have no opportunity costs.
Everything else is scarce. This includes sand at the Sahara desert. To be an economic good it needs to be put to use. If you are in the Sahara desert, and you are using sand to make glass, then sand is scarce. The reason is that there is an opportunity cost. Every decision has an economic cost of time. Time is an opportunity cost sacrificed in each decision. Of course, also the opportunity costs would include other possible uses for sand other than making glass, and whatever facilities you are using to produce glass could be used to produce other goods. Opportunity costs arise from scarcity.
Human’s also do not innately trade things. This is the point that I took most umbrage to. According Yuval Noah Harari author of Homo Sapiens: A brief history of humankind, and almost every expert in the field, believe that humans ability to collaborate due to unique forms of communication is what gave homo sapiens the ability to populate the globe. The entirety of human history does not encompass humans trading, and this trading is not what gave rise to human development. For the majority of human history, humans worked in hunter and gathering groups, working together to provide for the community. Everything was shared, and collaboration was important to survival not competition.
ECONOMICS IS NOT THE STUDY OF TRADE. Economics is a discipline that analyzes the distribution of resources given scarcity. It can happen through trade. That is neo-classical economics which focuses on market mechanisms, and the exchange/market of goods