Most repair shops and dealerships make a profit on the parts they use in a repair. The good news for consumers is that more and more shops are willing to install parts purchased elsewhere. A recent phone AutoMD.com survey of more than 125,000 repair shops revealed that nearly half were willing to install parts brought in by customers.
But, be sure to check with your shop before purchasing your part. And keep in mind that most shops will only guarantee labor if they make a mistake, not if the part fails or is defective in some way. Ask what the policy is before you commit.