The argument has to be that systems that don't have private ownership get bogged down quickly in waste, inefficiency, and corruption: that if it's no one's property, no one has enough motivation to watch over it--and you get large-scale environmental disasters as the tragedy of the commons takes effect.
I'm not sure the argument is right. But that's what the argument has to be.
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>second question: i work in a volunteer shop that works pretty much like oxfam
>shops. all the things we sell are given to us for free. when i put a price tag
>on something, am i creating money ? if not, what am i doing ?
You are creating wealth (as long as someone buys it, that is). Money... money is a particular form of wealth: it is wealth that others will accept in payment. That's why we tend to say that banks are the only things that create money--because the checking account deposits that they credit you with are accepted by others as payment for stuff...
Brad DeLong