One of the side quests the player (Commander Shepard) can undertake is Mass Effect 3 is one that demonstrates the old idea that trade only occurs if both parties agree. During Mass Effect 3 (remember, galaxy in chaos, massive invasion), Commander Shepard becomes aware that a refugee group has a large supply of military-grade medication, exactly what one needs to fight and win a galactic war. Commander Shepard goes to the refugee leader to discuss acquiring the medication and offers up his surplus of food, exactly what one needs to feed a refugee camp. Initially, the refugee leader refuses the bargain, trying to get more out of the Commander. However, when Commander Shepard goes to walk away from the deal, the refugee leader backs down and agrees to the original exchange. The refugees get their much-needed food and the military gets their much-needed medication.
Both parties here had an excess of goods and were able to trade it for supplies they needed/valued more. Both parties were made better off after the trade than before because both parties could now consume more. The galaxy was made a little better, a little wealthier. These sort of bargains go on all the time in real life, to the point they are so common no one notices.