Just as general information, mechanics do not like to install parts brought in by the customer because of potential problems with "comebacks". If the part fails, the customer usually expects the mechanic to warranty it, even though he select it, purchase it, or make his normal mark-up on the part. Plus, if the part doesn't fit, can he charge for his time to get the correct one?

Even customers who agree to accept this risk up front often balk at having to pay additional charges when something goes wrong. Since the mechanic is in the business to make a living, and has probably been burned before this way, there is not much incentive for him to take the risk.