? how does one pay for a good, without money?

Well, as of right now, the only way to pay for a good is to pay for it with another good or service. As before, money is merely the accounting of debt. It only signifies a promise to provide a good or service in the future. You aren't really paying for something with money, you are delaying payment with a promise to actually pay later.

Given that, why do you think debt is necessary in the hypothetical future situation that was presented?