"this is a serial number for a product that has been replaced"

I am trying to buy a Apple Pencil from some person but when I add the serial number to check the device's coverage session of apple website, it shows "this is a serial number for a product that has been replaced." What does that mean? I was looking at other people post with same issue but the response was "that likely means they were reported as defective and Apple replaced them but they were not returned to Apple as required" so does that mean it was stolen or the product was damaged or its not real?