Why bother selling it at all? You could just give away the things for free. The marginal cost of an item is just the energy needed to produce it. With robots building solar panels or something like that, those costs are negligible.
The idea that you could do something like that is even more outlandish than having robots that could automate everything a human could.
But I didn't say this.
Certain materials can be substituted for another, but for high energy costs in making them.