There is ample literary as well as archaeological evidence of trade links
  between the Sumerian and Indus people. The Sumerian texts refer to trade
  relations with ‘Meluha’ which was the ancient name given to the Indus
  region, and they also speak of two intermediate stations called ‘Dilmun’
  (identified with Bahrain) and Makan (Makran coast).
  Discovery of many Indus seals in Mesopotamia and evidence of imitation
  by the Harappans of some cosmetics used by the Sumerians suggest that
  some of the Harappan merchants must have visited and resided in
  Mesopotamia. About two dozen Indus type seals were also discovered
  from different cities of Mesopotamia like Ur, Kish, Susa, Lagash and Tell
  Reciprocal evidence comes from the Indus cities also—discovery of three
  cylinder seals of Mesopotamian type, a number of metal objects of
  Mesopotamian origin and the pot-stone fragment of a hut-pot at
  Mohenjodaro; discovery of a circular button seal (which belongs to a class
  of ‘Persian Gulf seals’), several bun-shaped copper ingots of
  Mesopotamian origin and the ‘reserved slip ware’ of the Mesopotamian
  type at Lothal; discovery of the ‘reserved slip ware’ at Harappa also–all
  these provide conclusive proof of trade links between the two people.
     There is absolutely no idea about their currency. All exchanges were
probably carried on through barter.
     Trade was carried on by overland as well as overseas transport. Bullock
carts (evident from terracotta models) and pack-oxen were employed for land
transport. There is evidence of sea and river transport by ships and boats in
several seals and terracotta models, apart from the dockyard at Lothal.
Several representations of ships are found on seals of Harappa and
Mohenjodaro and a terracotta model of a ship, with a stick-impressed socket
for the mast and eyeholes for fixing rigging, comes from Lothal. Of the
inland travel on the plains there is plentiful evidence from terracotta models
of bullock-carts. From Harappa and Chanhudaro come copper or bronze
models of carts with seated drivers and also nearly identical models of little
carts of the modem ikka or ekka type, still common in the Punjab. These have
a framed canopy over the body        in which the passenger sits. For longer