I agree that "greediness" is the quality of being greedy, and "greed" ought to be a more direct word, but in practice it's not easy to think of sentences where "greediness" could not be replaced by "greed" with effectively the same meaning. "greed" is a much more common word, so replacing "greed" by "greediness" can make a sentence sound less obviously natural, but, again, the actual difference in meaning usually seems minimal or non-existent. Perhaps an example of a difference might be when describing a person's propensity to eat large amounts.
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