I have been buying parts from them for years - probably since they started.
That being said, I do buy many parts local, but the more expensive stuff I do buy online - What can I say, we're on a budget.
To summarize what they do (or maybe their old model, as I have understood it);
They buy up inventory from warehouses/businesses that shut down, and sell them at great prices.
Their system is flawed just like any other catalog, so sometimes you need to go into a search knowing what you're looking for (case and point is a hydraulic clutch for a 1991 Grand Am 2.3L - most catalogs & applications try to sell you the Izusu hydraulics instead of the correct Muncie hydraulics).
This thread should be of interest to you:
ROCK AUTO DISCOUNT CODE