Why did my brand new part look used?

1. 1
   returning or refunding the part

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2. 2
   inspecting the part for signs of prior use

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3. 3
   straightening bent fins with a small flathead or radiator rake

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4. 4
   Checking if the panel is firm by pressing on it

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5. 5
   Pouring vinegar on it to check for bubbling calcium precipitates

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6. 6
   Relying on professional experience from installers to verify if the appearance is normal

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7. 7
   Test the part for continuity using a multimeter before installing

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8. 8
   Install the part anyway and see if it works

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In their words

Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.