i've done this for a while without any trouble. for parts, i charge whatever it costs me to get them, including shipping, but with no markup. i provide copies of the receipts to prove it. i give the person the option of either ordering it online for less money or getting it RIGHT NOW for more money. some people want it right away, others don't mind waiting.
i charge $50/hour for everything, with a 1 hour minimum. i offer no additional warrenties beyond that of the parts and negligence on my part. so if they screw it up, it's not my problem, and they have to pay to have me back. i do not troubleshoot over the phone beyond giving estimates. so i'll tell them what the problem is likely to be (and what it is likely to cost) but not how to fix it. for that, they have to employ my services. i'm also very specific about what services i have performed on a given computer so that there's no bull when they call me to say that when i installed their kid's game it made their router break.