I can answer two of those. Middle-men are useful whenever there is fan-out or fan-in. A producer wants to sell to consumers, so there is a lot of fan-out. The p

I can answer two of those. Middle-men are useful whenever there is fan-out or fan-in. A producer wants to sell to consumers, so there is a lot of fan-out. The producer could get the consumers to come to the factory and to buy good directly, or they could go door to door, or they could use middle-men: retail stores. The stores buy the goods wholesale and take on the work of selling them to individual customers, in exchange for a portion of the total profit. Customers visit stores for the same reason, to avoid having to visit every factory for every producer of the goods they want. With time and effort saved on both sides, the stores benefit both the producers and the consumers. The economy is not a zero-sum game because we are summing the value, not the prices, of the goods that are traded. If I grew wheat and you grew apples, then we can trade between us to mutual advantage, because after the trade we can both make apple pie (assuming we can find some cinnamon). The total amount of wheat and apples hasn't changed, but their value has gone up. Every trade involves two parties who must both believe that they will gain value by the trade. Of course it is possible for either party to be mistaken (or even deliberately swindled by the other party), but as long as the majority of trades have net positive value then the economy as a whole will be a positive-sum game.