I sure don't think I would ever consider sharpening as a business. It just has too many variables. I know many make a good living from this, it just isn't something I want to do.

I say that because I have been sharpening knives for 50 years and have a lot of sharpening tools that are normally great at sharpening. There are however, knives that just seem to not want to take an edge. There are many different steels and many different designs of blades and some are really tough to sharpen.

I have a SOG folder that a friend gave to me when I was deploying to Desert Storm. I like the knife, nice shape to the blade and it was sharp for a long time. When it first became dull I tried everything to get it sharp again. It is marginally sharp now, still nowhere near as sharp as I like a knife to be.
It has a stud on the blade, which can be unscrewed and removed for sharpening. Something about the steel makes it hard to sharpen. Very hard Japanese steel.

I am not a fan of serrated knives but they do have there place and are good cutters for many things. They are also hard to sharpen, especially partial serrated blades.

You would have to become proficient at all styles and types to make a decent wage sharpening. You couldn't pick and choose what you would take on, businesses can't work that way.

So, I try to keep my knives sharp, the way they should be to be useful and I do that at my leisure. I do sharpen for family and friends, if they ask. Sure don't want a career at it.