ADI sells various brands of cameras and some of them are sold online by B&H Photo for less than ADI's dealer cost. In one case, B&H had been on the manufacturer's list of unauthorized re-sellers, so B&H started buying them through a distributor who then negotiated a better price through their volume of purchase and that allows B&H to sell below ADI's cost and still make money. It's not only people selling out of their garage or basement- look at the manufacturer's unauthorized re-seller list and if someone questions your pricing, show it to them.

BTW- read the new NDAA changes WRT security systems and cameras- some well-known brands are no longer authorized by the US government for government use.