[jcp123]

Had a few years in at AutoZone. Some thoughts:

Shops get a discount but not huge. 20% at most, more like 10%. Markup was usually double their cost, so a starter which retailed at $100 would go for $90 to the shop, and they would charge the customer $180 for it. My experience is that if you bring in your own parts, you lose any labour guarantee.

Brake pads were mentioned: no, there is a difference in brake pads. I felt no sympathy for guys who bought the cheap crap. I wouldn't buy the cheap crap pads unless I were reconditioning a car for sale. The higher prices pads had bona fide advantages.

Yes, we fairly regularly supplied competitors' phone numbers; in the long run, that all evened out.