Having the confidence to charge the correct markup, deny customer supplied parts, and not compete with but beat mail order parts is the key.
If you call your local dealership parts department and ask for the price on a water pump it will be list price. If you call the same dealership service department and ask for the installed price and then a breakdown you will find the part price is doubled. Nothing illustrates the difference in correct markup between the two industries and you need the same mentality and markup.
When we are told "I can get it for less" we state that we can too. Then we calmly explain that a local service department and a warehouse in the Midwest have very different business models. Our service model have a heavy investment in floor space, technicians, overhead, the local economy, and most importantly local taxes. We are an integral part of our local community and support local charities, the highway department, the school district, state taxes and levies, state labor regulations, state consumer protections, and the list goes on and on. Unfortunately, the out of town warehouse model and it's inventory of Chinese parts does not in order to increase sales for their shareholders. That's why you can't go into red Lobster with your own lobster. (The local likes his road plowed, his children to go to school, etc. and will be willing to support that).
When we are asked to install parts purchased elsewhere we explain we can't. Then we calmly explain the part markup supports the entire business and its employees as stated above. But the biggest reason is liability. When you supply a part not only do you forgo all consumer protections and warranty claims on the entire repair, you incur a liability most consumers aren't aware of. We have an insurance company and they require product liability coverage from our suppliers. If you purchase a Chinese part from an out of town supplier and heaven forbid someone gets injured when it fails who pays the claims? Your insurance company owns the right to sue anyone it wants to recoup claims including you, it's in your policy. If we use our part it is an easy claim because our insurance will pay it and then go after our supplier. If you hand someone a part and ask them to install it who is liable then?
Most people don't understand the intimate details of running a service department and probably shouldn't learn as they go.