Ancient sumer was able to develop trade with its neighbors because sumerian farmers grew a of food.

the answer is:

    The answer is: surplus.

Explanation:  Since the Sumerian civilisation developed between rivers in the fertile plain, therefore, thanks to agriculture, combined with the rainy seasons during the year, meant that they were able to produce large quantities of food. That food was necessary for life, and any excess food could be exchanged for some other good, which was in fact a precondition for development. Every surplus of food, growing bigger and bigger, meant an exchange for something else that was needed, and over time, it had grown to such an extent that this exchange meant wealth, power and dominance in the ancient world.

Of course, this exchange of surplus food for other goods contributed to the development of trade with their neighbours, which was a prerequisite for the development of the empire. Thus, a trading system was developed that included established trade canals and their development and promotion, as well as writing trade agreements with a wedge-shaped letter, for easier, faster and more efficient trade.