Historians believe that the economy of the Indus Valley Civilization was based on agriculture and the import of raw materials from other lands for use in local workshops. As is the case everywhere, the cities were connected with rural agricultural communities, and there is evidence that they grew sesame, wheat, barley, mustard, millet, cotton and field peas. They may have been the first civilization to grow cotton for clothing. source
It’s believed that agricultural goods: grain and other foodstuffs, lumber, cotton and livestock were traded, and that they were the mainstays of commerce. This is supported by the discovery of granaries. According to Gregory Possehl’s book, The Indus Civilization: A Contemporary Perspective, he posits that based on evidence of trade goods like carved seals for stamping (possibly for signing contracts), ornate beads, pottery, metal tools and carved gemstone handicrafts found as far afield as Mesopotamia, Central Asia, China, Iran, Afghanistan and Oman, it appears that Harappans were part of a vast maritime trade network, on which the economy depended. This theory is backed up by the discovery of a massive, dredged canal at Lothal, which is assumed to be a docking facility. source
One standard brick size was found in several cities, and Harappans used fired bricks for the extensive water collection and delivery systems they had, as well as for baths and underground sewage systems. They may have been the first civilization to create urban sanitation systems and to use wheeled transport. It appears that the far-flung communities participated in some kind of taxation system, based on standardized weights that were found. source Since we don’t understand the written language, it is impossible to know details about how the system worked, and historians can only speculate.