I don't really see a problem either, but it doesn't mean I feel 100% comfortable with it either... maybe because of the local culture of music stores behaving the way they do. Everything I sell is for minimal profit, or sometimes no profit at all (which I only know after the fact, beause of some tax issues i.e. it's a small chance but for the prices I sell them, I might even lose a little bit). I also first made sure no one in the area was selling something equivilent that was as good value.
I think the main difference is that you're living off doing while others live off selling. You might sell only stuff that enables you to preserve and present your work as good as possible (eg a pot of cork grease) or that enhances the quality of your repair (a new rag, a new case, maybe a box of reeds or a mouthpiece), or simply because the customer asked. You're providing "only" added value while a "proper" shop must sell gear in order to survive.