Trombone oil is used to lubricate trombone slides.
It's basically a light machine oil, but it's also a wonderful metaphor.
Trombone oil is very specialized.
Musicians who need it will pay a high price for the best quality of trombone oil.
And the total world market is a few gallons a year. (Actually, that's a made-up number, but this is a metaphor, so never mind.)
The point is that, no matter how excellent a trombone oil maker you are, no matter how fine your trombone oil is or how much you can charge for it, you're never going to make a really serious amount of money selling it because the total market is just too small.
I find this to be an excellent analogy that can be applied to many situations, and now you can use it too.