I'm going to try it. It can't hurt anything to try.
If your not prepared, it can. If YOU fry a component because you mess up, guess who has to pay for it? Some components are not cheap. If you really have the experience that you claim to have, you should know that.
If your charging tax on the item, you need to charge it on labor as well. That also means you need to register the business with the government so they can get the taxes. I don't think Big-brother will allow you to open a business with as many liability issues as a computer repair service.
{Edit} Like mammikoura, I would love to know the kind of problems you have "fixed". I doubt it's anything more than reformatting a computer due to viruses. Question for you, what is a physical sign of a failing motherboard?