Just tell them that you will not warranty parts that you don't supply. I live in a small enough area, that the only parts stores around are autozone, oreilly's, and napa. I get most of my parts from factory motor parts an hour away, and I tell my customers that the parts are way better than the garbage that the others sell just to keep them from supplying their own. I will occasionally get a customer that wants to provide their own parts, and I let them, as long as they know there is no warranty.

Sean Van Horn
Owner/Technician
Red's Auto Rehab
Cornville, Arizona, USA