[William]

I have seen a modern, professional, mobile knife-sharpener within the last few years.

I was living in Portugal. Every now and then I would hear an odd, tuneless whistling from the street below, but I was never sure what it was. One day, I happened to be leaving my apartment as the player came by.

It was a man pushing a modified bicycle. No longer ridable, the pedals instead drove a knife-sharpening wheel. He was playing a simple wind instrument of some kind to advertise his presence as he walked down the street.

He was clearly not wealthy at it, but it was also obviously at least a part-time gig for him. If he can make a living doing it today, I would certainly think someone could do the same in earlier centuries.