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...there's nothing wrong with that Shimano derailleur, from a functional standpoint. It will certainly do the job required. But it's a low end Shimano product, as you can tell from the general fugliness of it, and the stamped steel cog cage. The only justifiable reason for swapping it out, would be to convert your bike to indexed shifting on a budget. Which clearly has not been done.
I share the dismay of others on the used chain offer. I would not go back to that place.