why do people do this?

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I sell some stuff on Amazon. Products are new and factory sealed. I have 300+ orders with no issues whatsoever. I have 30 positive feedbacks (*****). Then suddenly I get a ONE START feedback saying product was USED AND DEFECTIVE. I finally get the product back, and it is obvious that the customer sent back a different item, which was abused.

The customer took advantage of me. well, nothing I can do now. But why did they leave the nasty negative feedback?