I generally think people selling niche books online just go out on a limb, because they can't be bothered doing the research. Most people impulse buy on the net and don't do the research either.
You could quite easily get rich collecting and selling photography books online, because the market has a solid foundation in ignorance.
Just looking at Paul Strand's Outer Hebrides, which I bought brand new for £50 last year. Somebody is selling the same edition for £871 on Amazon... and I bet it sells.