Sometimes you buy the parts, then finds that you are not going to have the time, tools, or ability to perform the repair.
For this reason, I prefer mechanics who are willing to use my parts to bail me out.
Generally, I would prefer that a mechanic not rely on parts markups for much of their revenue. The natural incentive would be to replace things that may be repairable. I would rather pay more for labor as needed. Just my opinion.
My favorite mechanic will even install used parts, if I can rustle them up. No guarantee of any kind on the 'my' parts is the risk I take. Fair enough.
That said, I understand the liability perspective in our litigious society.
1986 300E 5-Speed 240k mi.