Amazon stated (I believe) when it began that they sell books you cannot find. EBay I believe was similar, something to the affect that you could sell the hard to sell stuff.
This is the Long Tail story. Yet, as I look through the history I have seen a pattern emerge that I call Tail Spike - when a niche is cpatured in the long tail, the adjacent areas of the tail (to the captured niche) start to ebb while the niche starts to spike, creating what I call the Tail Spike.
Proof is Amazon, EBay, etc. that today are nothing close to what they began as. The Tail Spiked businesses are mostly online businesses that are unencumbered by the physical shelf that must hold the few products that will sell. The unlimited, ondemand virtual shelf can hold infinite products and services.
More to follow in the next few days.