Most of the bike shops selling online are just regular bike shops with some extra staff, and if they're lucky, some extra room.

At Harris we carry an inventory of everything you can get to from the table at the top of this page. Anything we have (that we haven't run out of at least...) gets shipped out the same day as your order, or the next business day. The internet department is here in the store itself, and the parts are the same ones we use in the service department, at the same price as if you walk in the store. You should see our shelf of 7 speed cassettes.

The QBP special order catalog is what Grand Bois is referring to, those parts get here and get shipped out every Thursday if things go right, and they usually do.

Any online retailer that's "a real live bike shop" selling things suspiciously cheap is probably very similar to what MDM is describing. Luckily that's not most of them, there's only a few.