El Maximo wrote:

The only full-time independent CFI I know if a retired schoolteacher.
Everyone else I know either works for a school, or only freelances
part-time.


Part time businesses are still businesses.

I've had them all my life. After a certain threshold, the income is
taxable. Once you hold yourself out, you get liability and
responsibility. Some jurisdictions may require you to colelct sales
taxes, register yourself, etc...

It usually makes no difference if the business is part or full time once
money starts changing hands.