You might think it is in the collective interest that Palestinian children survive. I would also want that. But most Israelis think it is in the collective interest that Palestinians are exterminated (as far as I can gather, let's not derail the conversation into that example).

My point being that the principle of collective interest is not grounded in anything real, it is an assertion of desire, perhaps widely shared but nonetheless not a starting point.

People can only be sure of their own desires. Markets inform them about other people's desires.