The usual answer to this is the cotton gin. It is said that Eli Whitney’s invention of the cotton gin in 1793. This invention revolutionized the American cotton industry. This eventually led to changes that allowed the Market Revolution to happen.
Whitney’s cotton gin made it possible for cotton to be produced in huge quantities in the South. When this became possible, Southern plantation owners needed large numbers of slaves. The number of slaves and the amount of cotton both helped cause the Market Revolution.
With these developments, factories arose in the North. They were mostly there to make the cotton into cloth. Some of the factories were also needed in part for making things that could be sold to the South, such as shoes for slaves.
With the factories, the American economy was transformed. People were no longer working as subsistence farmers, making most of what they needed and perhaps trading locally for what they did not make. Instead, there was now a large group of factory workers who were working for wages. There was also an increasing middle class that did things like selling things to the factory workers.
When this happened, America’s economy moved from being a subsistence economy to being one in which people worked for monetary wages and bought things using money. This is the Market Revolution.