Manage an auto parts store here. Company name doesn't matter in this case but I can give some insight here.

They have extremely small overhead costs. In a standard retail parts store you obviously have the cost of employee labor, the cost of the building rent, utilities, delivery vehicle fuel, lease payments, etc. Then there's the uncontrollable stuff from up above such as logistics cost for replenishment orders, marketing, customer satisfaction, hr, etc.

Your standard operating margin is roughly 40 percent. Now that sounds high, but it fluctuates so much depending on the product that it's hard to nail down. For example, I might make 40 percent on a caliper, but I can also make 500 percent on an air freshener. Air fresheners are obviously more retail friendly because it's an impulse purchase, it's low cost, and marketable. Then there's the logistics of fitting 100 air fresheners into a tote versus 4 calipers. If I sell 30 of those air fresheners I'll make more actual profit and dollars than selling 4 calipers and I still have another 70 air fresheners that will turn 10 to 1 to a caliper because they aren't vehicle specific.

Rock has none of the retail stuff, so they have no fast turning inventory. Almost everything they sell is vehicle specific. So they have warehouses, pickers, shipping contracts, purchase contracts, and your standard business stuff like hr and payroll. That's it. Very low operating costs comparatively.

Another aspect that as a parts store we have to tread everyday when someone asks us to price match rock is warranty. If you buy a caliper from me and it locks up, 95 percent of the time I can have another 1 same day. With rock your waiting. Their warranty process sucks and they have full say on whether or not they'll honor it. Parts stores expect stuff to come back, it's the nature of the business. That warranty cost is figured into the retail cost essentially. It's usually an aspect that most people don't consider until something happens.

Tldr; the business models are different. Parts stores cater to a much broader customer base but the costs are higher because of the convenience factor. Rock is essentially a wholesaler with a catalogue and really low operational cost.