I just received a paperback with a burn mark on the upper edge of the pages.  It looks like perhaps a cigarette ash fell on the book and was brushed away - it is about that size.  It cuts into a small chunk of pages but does not obliterate words and does not compromise the structural integrity of the book. 

Postable, or not postable?  (This will inform whether or not I mark it RWAP...if it's unpostable, I will want my credit back!)  Thanks.

Well, usually dirt or smudges on page edges can be removed with light sanding with sandpaper. So, if I am able to remove the dirt or smudges, I do not mark RWAP.

Technically, books cannot be dirty or stained, though.

So, I guess you have to decide!

Its a stain so I would mark it RWAP..Burn marks make the page edge brittle too.

I've seen burn marks on my mother's books before (she's a smoker). If it's the kind of burn I'm thinking of (more of a burn and less of an ash stain), I'd RWAP it.

But pictures would help us to judge better.