When I lived in Lynchburg, Va., I shopped at Hammersley and I ordered parts from them after I moved away. They were small dealership in the back of a BMW, Mercedes, etc. dealership. Since they were a small dealership in a small community, I think they survived by selling parts at a discount throughout the country. They made less profit on the parts they sold at a discount but it was profit and sales they would not have had if they had only sold at MSRP.
Selling parts is no different than selling anything else. Your business model can be selling fewer goods at a higher profit per unit or selling more goods at a lower profit per unit. As we learned in Econ. 101, the law of diminishing returns is a delicate balance.