Here's my experience with O'Reilly's: I think they realized that dealing with a customer base that's looking for wiper refills or complaining about a $5 price difference on a master cylinder is nothing but a race to the bottom of the barrel.
Several years ago a local sales rep came by and explained that they had recently finished a huge warehouse/distribution center nearby and were wooing the professional market. They largely got rid of their house and economy brands and were now stocking industry recognized brands for the majority of their parts. I toured the facility and found it to be well-stocked. They also installed "back counter" or wholesale only people in the store that serviced me, so when I did need to phone something in, I would be dealing with a seasoned pro and not a college kid with a part-time job.
You see, those seasoned pros at the parts counter are too valuable to the wholesale accounts to be dealing with DIY Bob who comes in without a VIN or RPO code for his car but wants the correct brake pads and pulls out a rewards card to make sure he gets a lifetime warranty.
Of course the quality of any vendor is as much dependent on the people that work there as the parts they sell. I can say my wholesale vendors from O'Reilly's have consistently gone above and beyond to get me what I need to fix the cars in the shop.