Forgot to say that I'm in Ireland not USA so things are slightly different regarding pricing but your points are still valid. Liability is with us if anything goes wrong.
Just recently we noticed some websites are shipping parts from Germany for significantly less than we can buy. We said this to our parts supplier and they can't match the prices either. This is just the way things are going now, so we made a decision to meet it head on.
There is a national vehicle Road worthiness test here so customers wait until the car fails the test and then have a list of faults to rectify. If they buy parts to rectify the faults from our supplier we will carry out the work at a slightly higher labour rate.
We won't let them supply parts from any other source.
Without a test fail sheet, we offer a vehicle check and let them make an appointment to get the parts fitted at a later date. We charge for the inspection and again a slightly higher labour rate.
Once they have been cleared by us to buy parts from our supplier, we get a cut of ANY parts, accessories etc they buy