Before the advent of coins, how were small transactions such as buying street food handled by everyday people in bronze age Mesopotamia? I feel like the barter system would be a little cumbersome and impractical for something so small, and can't think of any other method by which somebody could go about getting a snack.
A secondary question is whether food stands like you would find in forums and marketplaces even existed in a recognizable form in the bronze age.
Currency in general was developed as a means of simplifying trade in valuable goods. Bartering is currently believed to have not been quite as common as pop history suggests due to the "coincidence of wants" making it impractical for many civilizations. A valuable good, such as grain, would come to be a standard method of trade.
Sumerian currency developed as a way of simplifying trade. Currency like the shekel is referring to it being worth 1 shekel of grain, which was a measurement of weight at the time. Early Sumerian currency was in the form of clay tokens in the shape of a particular good like livestock or grain, which acted as representations of the real thing in storage elsewhere. It's much easier to give someone the tokens than cart it all to the market in one go! It's even theorized that writing developed as a way of measuring value: they would seal the tokens in a clay container and need to create symbols to write down how many tokens and what kind were inside.
A lot of records from this time period are scarce and historians need to make educated guesses, but it's also believed that jewelry would have been used as a form of currency if necessary (being made of precious metals and stones). They would later develop silver coins to act as standardized currency: the shekel, the mina, and the talent.
As far as buying food from a stall, by the Third Dynasty you are in a world with coinage, specifically one that can be subdivided. Legal documents like the Code of Hammurabi set the wages of labor and an unskilled laborer could expect a wage of 1 shekel a month usually, in addition to a grain ration from the government. Merchants and craftsmen selling their wares could be paid in either these standardized weights of metal coinage or jewelry or in something of equivalent value like grain.