>which is severely handicapped for the candidates, because they don’t know the offers made to others in comparable experience and education
It does not matter. If you are not satisfied with the offer - ask for more or walk away. If you are - then it does not matter what others got especially to you, who have not evaluated them at all. What you think is "comparable" might be far from that in my view and vice versa.
>In some labor markets, the non-negotiable salary is advertised with the job ads. Not claiming that is an infallible solution, but it makes for a drastically different labor market.
Yeah, I gave an example in the post you replied to: the unskilled labor market. Amazon pays the same rate to every warehouse drone and it's known in advance. Same as many retail jobs, same as many trades.