If you buy actually valuable expensive input from the outside world for making food, you should sell actually valuable output to make it worth it. Otherwise it isn't sustainable.
Sure, making money on things that aren't actually valuable is all too easy, but that would defeat the point. "Economy", formal and informal, seems to be a big and acknowledged part of permaculture culture.
If "permaculture" meant "hobby farm", then you'd be correct.
However, permaculture is centered around a radically different approach: maximally leveraging existing ecosystem features rather than trying to fight them, or trying to wipe the ecosystem clean and turn it into mere substrate for building something artificial.
A correctly implemented permaculture "farm" should require minimal or no artificial inputs at all, instead relying on a composition of self-sustaining closed loops (e.g. food waste goes into compost and a worm bin, where the compost is used to grow more vegetables while the worms are used to supplement chickens foraging in the garden and eating pests like slugs)
Even if that, you need shovels. You need a lot of stuff that wears out, that you can't make yourself. You need to participate in an economy. Some of it can be with other permas, but much of it likely won't.
It's always been like that. Farmers from the stone age on have used tools manufactured a long way away.
Sure, making money on things that aren't actually valuable is all too easy, but that would defeat the point. "Economy", formal and informal, seems to be a big and acknowledged part of permaculture culture.