That is an interesting question. I was talking to a local vintage photo shop owner and he shared that he routinely applies a little cosmetic touch-up to his lightly-used items. Examples:
- Using a black Sharpie (marker pen) to dress bright knurling on focus and aperture dials
- Using shoe polish on leather cases
- Using a lacquer pen/crayon to refresh embossed lens/body lettering
That last point probably explains why my Vivitar 135/2.8 (purchased from said dealer) has red metric scale marking while most other examples I have seen have green
I use the same tricks with my gear, though I should add that I seldom, if ever, sell anything. I should also add that the touch up is very difficult to detect and in the case of my two XR Rikenon 50/2, essential for usability. (As is typical for that lens, the lettering was flaking off with age.)
What I do not appreciate is a "quicky" recon job on gear that should show some evidence of wear. This last Spring, I bought a Kiev IIa from an established dealer and was surprised to find that the leather cover had be recently replaced with leatherette and the black surfaces recently brush-painted with black lacquer. The paint was still sticky. My expectation given the item description would be that the leather be original and intact with a few "Contax" bumps and that the painted surfaces should show some corner wear. As it turned out, the camera had functional issues and was returned for full refund, so no harm, no foul.
So...What to do? What you choose to do with an item intended for sale depends, I guess, on how you want the buyer to react when/if they discover your touch-up work. If you think they might feel cheated, don't do it.
Steve