That way both of us will be better off than under the strict egalitarian version. Granted, the bigger the market, the more likely I am to find my trading partner. Still, illiquid markets or small ones inhibit the scope of these exchanges. This is the price of the oranges in terms of books. Money makes matters more convenient - but it does not eliminate the necessity to negotiate.
Market failures abound.
It is merely a medium of exchange. Thus, indirectly, trading and the market are instruments of increasing happiness and welfare. The invisible hand is also a just and benevolent one. Money is a medium of exchange which increases happiness because it facilitates exchanges, makes life easier, increases our welfare by allowing us to get exactly what we want. We trade what we do not want, nor need - for what we both want and need.