Along with what 180 said, I have been told by so-called 'mechanics' at dealerships (during bar round buys and overhearing conversations at the next table) that dealerships trade out parts and sell them to others and make a huge amount of money. They are supposed to be returning the parts to someone but on many occasions they just disappear never to be seen again until installed in another car on the sly. Given the history of some bad apples, I would tend to agree to an extent.
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