I have just one word for you to remember. "LAWSUIT"
The first time some fool gets you to sharpen a blade and then they hit a rock and it flies out and hits them you could have a lawsuit on your hands. Do not laugh about that. It happened to my boss a few years back. He sold a Echo trimmer to a man. A Echo metal blade was installed when the man bought it. The man takes it home and hits a rock with it and a bit of rock and/or blade hit him on the leg and cut him. He filed a lawsuit against my boss. Echo picked up the lawsuit because a Echo blade was installed but if any other make or brand blade had been use Echo would have stood back and let him take care of it the best way he could. So remember all of that that can happen. And how to you plan to get rid of the used oil and gas? If not done right which can cost a lot the EPA will be all over you. And what about insurance in case something left at your shop gets stolen or your shop burns down or something else happens to the machine while it is in your care? That is way I work for someone and gave up on the idea of my own shop.You can advertise for any thing you want just be sure you are not advertising for trouble.