it int easy that's for sure.

You have to find a niche market and then keep it to yourself.

I make a sell a widget. Other people make a similiar thing.

But - as far as I know - I'm the only person who sells them in a shop friendly packet with everything necessary for the use included, also mine is self adhesive and nobody elses is :-)

I designed and printed the machines I use to get a perfectly sized and centered tape on the widget.
That's what 3d printing is best for.
Making tools and jigs to help in making things you can sell.

But it's not certain or easy to make money with cheap 3d printers. Anyone who tells you it is, most likely is trying to sell you a printer :-)