Theory:

Traders had trade contacts in the country and also with other countries such as, BabylonPersia, Mesopotamia, Afghanistan and Egypt.
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Inhabitants of the Indus valley civilization  also trade contacts with Mesopotamia, southern India, Afghanistan, and Persia for gold, silver and copper.
traders of Indus.jpg
 
The estuaries, also known as the mouths of rivers, are home to Shallow Harbour. The Mesopotamian cities were able to engage in successful maritime trade thanks to the river's opening into the sea.
 
maritime trade.jpg
Indus Civilisation is considered as the first civilization to use wheeled transportation. Advancement in transportation technology lead to the invention of bullock carts and boats.
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Most of these boats were small and had flat-bottom. A dredged canal was used as a docking facility at the coastal city of Lothal in western India, which helped in exchanging goods with the people of other civilization.
 
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The people of Indus used camels, oxen and elephants to travel overland. They also had bullock carts with wooden wheels attached. They also had ships with one mast which is believed to be as sailed around the Arabian Sea.
 
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Some of their trade networks encircled a wide area, including some parts of  the coastal regions of Persia, Mesopotamia, Afghanistan, western and northern India,  and the parts of Egypt and Crete.
Currency:
During the Indus Valley civilization, there was no metallic money.
 
barter.JPGbarter1.png
All of the trade was conducted through the Barter System, exchanging goods or services for other goods or services without using a  currency.
Reference:
https://www.harappa.com/sites/default/files/styles/galleryformatter_slide/public/slides/indus-trade-networks.jpg?itok=HC3gL90l
Reference:
https://www.harappa.com/sites/default/files/question/indus-life.jpg
https://www.arcgis.com/sharing/rest/content/items/c51568ed22a94cec8625159006860eed/resources/lothal-slideshow-image__1512505790151__w600.jpg
https://ichef.bbci.co.uk/images/ic/336xn/p02mqs7m.jpg
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https://www.harappa.com/sites/default/files/styles/galleryformatter_slide/public/slides/lothal-2.jpg?itok=0puooDZw
https://static1.squarespace.com/static/5a7e1ecc010027458112449e/t/5ada3f440e2e721d2b5fc255/1524252491576/Indus+Valley+Civilization+%E2%80%94+Livelihood.png?format=1500w
https://res.cloudinary.com/dk-find-out/image/upload/q_70,c_pad,w_1200,h_630,f_auto/A-Corbis-LL002185_phf1cs.jpg
https://www.netclipart.com/pp/m/120-1200113_humans-began-bartering-as-a-form-of-early.png
https://sites.google.com/site/mansivoraseportfolio/_/rsrc/1346761988950/barter-system/baretr%20system-1.jpg?height=176&width=320