I bought cheap "leather" case from ebay. After I noticed this thread I checked my Nook and it has these marks
It is a first time a have such issues with cheap ebay cases.
It looks like it is a stain not a rubbing off of nook plastic.
Probably it is a black dye from fake suede used as liner migrated to the nook plastic.
Plastic used in Nook definitely is a factor.
I wish I noticed this thread before I used the case. I ordered new one from BN.
I would greatly appreciate if somebody shares a way to remove these marks.