I've done the cost analysis and even with "free" hubs (i.e. I already had them) the cost of a complete wheel is the same or less than buying just the rims and spokes as an individual The big dealers/sellers buy so many that their cost is so much less than ours can possibly be.

One possible explanation for why hubs are so cheap on E-bay is that the owners have made the same discovery. They can't afford to build them up compared to buying complete wheels.