My conversation is generally based on "if something goes wrong with the car will you expect me to help you with the cost of repair?" Normally the answer is yes and so I confirm "If you take all my margin I will have no money left to help you with therefore my price is fixed". If the answer is no, I briefly outline their rights under the CRA and let them know that it is not an option for us to duck our responsibilities, therefore the price is fixed.
Asking the customer if they would work all week for less pay normally works equally as well.
Quality cars, fully checked and warrantied are worth the asking price.