If you have a book that is in great condition, even the pages - except the inside of the cover has that yellowish tanned stain either starting to appear or in larger form - would that book be considered non-postable?  I've always thought of stains as unidentifiable yucky stuff, as opposed to normal aging.  What say you?

I consider it normal aging.

Yep, me too.  The Help Center specifically says that yellowing on the pages, from aging, is acceptable, I can't see why the cover would be different.  (and I have never had any trouble with sending them)

Tanning will be even across the entire page or cover, or will gradually fade from the edge which will be a bit darker.  If the stain is irregular in shape at all, it isn't tanning.  The former is postable, the latter, not.

It's caused by oxidation with aging and is a very common phenomenon with older paperbacks especially.  I have a lot of books like that and they are quite postable.  Most of the ones I got in a 10 for $1 deal at a garage sale over the weekend are like that and I have listed them all.  I may send the requester a "heads-up" PM if the one they ask for is markedly tanned/yellowed, but still send it on.

"or will gradually fade from the edge which will be a bit darker"

This is what came to my mind when I read the original post.  I can see how it could resemble a stain even though it is actually normal aging.