The other place where you can earn huge margins is with newly released equipment. Look at the prices of stuff when it's first released, compared to a few months or a year later and you'll see that in the UK the must-have-it-now crowd pay a big premium.
For example, from memory the Canon 70-200/2.8 Mk II was £2799 at launch, but now it's £1900 or so. And the Canon 24-70/2.8 Mk II was £2299 or £2399 at launch, but now it's around £1800. Somebody's making money there!
I think the dealers make big margins there, partly because they can, and partly because they need to cover the cost of holding stock of a large range of lower margin products. But if you set up a niche business without such a large range...