Under the barter system of exchange, acts of sale and purchase of an individuals must occur at the same point of time. To buy a thing, an individual must at the same time sell something needed by the other person. Also, sale and purchase by an individuals must be of equal value.
With the introduction of money (as a medium of exchange), an individuals can buy a thing with money without selling anything at the same time. Likewise, he can sell a thing with money without buying anything at the same time. Thus, with the introduction of money, acts of sale and purchase have been separated.