Many people think you get what you pay for. Don't string for free, unless that is what you want to do. I would go for certification, that shows everyone that you know what you are doing.
When I first started stringing most people wanted me to string their rackets. is there some reason you can think of that would make people not want to have their rackets strung by you?
Everyone has the right to be stupid... it's just that some people abuse the privilege