I've noticed that sometimes about Green, I must say. But the point is that if Brown has money, then the problem is alleviated because I will always want to trade for money, in some amount. We may both get bored trying to figure out how to work our trade, we may have to involve third parties in a triangular swap, and in any case we expend time and energy in trying to resolve the details. Now take cash. . . . Gladly, and in whatever denominations may suit you, White.
Cash has a number of characteristics that make it desirable.
In the first place, cash is self-authenticating. You don't have to run the risk that the cash you accept from Green will be refused by another trader. In fact, cash is quite unusual and enjoys a favored place in our system of property. If you steal Green's gold watch, and then swap the purloined watch to me in return for one of my inspiring commercial law seminars, then Green can recover the watch from me in a legal action. It doesn't matter that I believed that the watch was really owned by you and that I provided the service in good faith and with pure heart.