"Not all will have it, or like it, but many will."
That's pretty much a solid definition of a niche product. Smart phones are a niche products. Anything that does much less business than that is a failure.
Huge profits can be made on niche products, so I'm not at all surprised that Amazon is making a killing on them.
$350 is a lot for me as I don't do a ton of leisure reading, nor make a lot of money after bills.
In any case, I think we're just arguing over semantics as you have a wrong definition of a niche product as something hardly anyone buys and that makes little money.
A niche product is just something that's not mainstream, not something that's not profitable or successful.