Just out of curiosity, I wonder what the one blade you couldn't get sharp was? I always had trouble with stainless steel blades, like they have on Swiss Army knives. They come razor sharp, so it must be possible to sharpen them, but they just frustrate me. I won't buy them anymore, except for serrated edge blades for the kitchen, which I also can't seem to sharpen, but I'll use them for about ten years and then just buy another one. I'll bet if you offered to sharpen stainless and serrated edge blades, you'd get a lot of business. A new higher end Henkels bread knife costs about $40-50, and if you could get it back to original sharpness for $10 plus shipping, I bet you'd get a lot of business.
OTOH, I doubt if many people who buy $200-$300 custom knives want anybody else to sharpen them. But then they come with steel that is pretty easy to sharpen.