The process of creating them is not free (at the least it will cost time) and for some people, depending on their skill, it's not even possible to customize CSS at that or any level. The style selection is unparalleled when compared to other button packs.

Why not just create a CSS generator. That helps someone tweak there button down to the very last detail, and then just charge them 50 cents for that one button.

There are already CSS button generators (for Bootstrap, even) and it's always the same: you get out what you put in. Even the examples these sites use as the buttons you can generate typically look like a lousy lot. And $0.50 would end up being like 80% credit card fees for me. It probably wouldn't work out.