[Ripplinger]

Possibly you had a bad fitting cover that rubbed excessively against the reader. I have skins on my 350s, and there's not even one spot that shows any wear at all as if the cover would be rubbing for either the standard or lighted cover (I have both). The skins looks perfectly new all along the spines.

If it's just from the plastic center black bar, I would think simply taking anything a bit gritty like extra fine steel wool and rubbing lightly would remove those black marks easily. Of course take extra care so you don't get any steel wool near the screen to mess with the infrared.

In hindsight, it probably would have been best to take pics of both sides of the reader clearly showing any type of flaw, no matter how small, and giving an explanation of what it's from.

Edit: Probably even an eraser would remove the marks if it's really from rubbing against the black plastic of the cover.