There's also asymmetric information with the milk. I don't know what it cost for the producers, or how much the store paid wholesale. Maybe it's only $0.05 a gallon on their end. Who knows? The only thing that matters is if the retail price is something I can afford and am willing to pay.

It's true that employers are privy to a lot more salary information than employees. I know I said what other people are paid is irrelevant, which I think it is in terms of fairness, but you should be (and, legally at least, in the US, are) free to take that information into account when deciding what you'll accept. It's still up to you to make that decision and hold employers to it. They aren't going to offer up higher salaries out of the kindness of their heart or some sentimentality about fairness.