I'm good at blowing bubbles with bubble gum. I have yet to charge anyone for doing it.

I suppose you could say that as long as I gain pleasure from blowing bubbles I'm not doing it "for free" but that makes the statement very trivial. Under normal interpretations of "for free", the statement is wrong because there's no demand from anyone else that I blow bubbles.

I'd correct that statement to "if you're good at something, never do it under market value", which raises the possibility that I would still do for free things like blowing bubbles that have no market value.