"Honesty is one of the rarest kinds of wealth any one anyone can possess , (comma here) and yet all the honesty in the world ain't isn't lawful legal tender for a loaf of bread" The author is saying that honesty is extremely valuable, but you can't even buy a loaf of bread with it. So it's not necessarily least appreciated, but it doesn't help you to get anything tangible, like food that you need to live. It's similar to the line "Money can't buy you love".
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