Just to add to your example list, as I house painter, I pay perhaps 50% of what the major paint brands/retailers will charge out on the shelf for most of the products that I buy. I don't have to throw crazy 'volume' at them, but honestly, I'm sure any painter buys substantially more paint in a year than any homeowner.
They provide tiered pricing -- If I tend to buy a lot of xyz, then I get better pricing on it, etc.
Being a reasonable folk, I only mark up a handful of % off my price -- I hunch many in my industry do a similar markup off retail.
Pricing is a very strange world.