Hard to say. Could be a broken spine and liquid damage. 

A good rule of thumb; If you're not sure, just don't post it.

The spine issues would only be a RWAP if the pages are coming loose as this kind of wear is typical in most newer books.  If the orange is a liquid stain then yes it would be RWAP, but not if it is just marker or pen (possibly a remainder mark or store discount mark).

Edit to add:

Here's a bookmark of the cleaved spine topic I tend to refer to on binding matters:

http://www.paperbackswap.com/What-Cleaved-Spine/topic/150725/

On 1/27/09 7:49PM, Kim received a great response from the PBS team clarifying this.

"Dear Kim,

We would not consider a pinhole to be a cleaved spine. Books that fall open to a specific
page are postable, and almost every paperback book that does that has at least some small separations.

What matters here is if the pages are securely attached. If they are likely to fall out on the next reading, then the book should not be posted. We really cannot clarify this any further. We need members to use their common sense and best judgment about whether a book is about to fall apart or not. This is just not "something we can judge without having the books in our hands."

Could the orange stain be a marker of some kind?  It sounds a lot like a remainder mark, as Melissa suggested.

If I had a really hard time deciding whether a book was a RWAP or not, I'd decide it wasn't.  JMHO.

The spine description sounds like a no, it is not a problem. It sounds like "pin-hole cracks" and is perfectly postable. As long as the pages are still attached to the book, a creased spine is never a problem, the book can open to a certain page, and there can be small areas where the glue is inconsistant.

The stain is not something one can determine without being the person holding the book. If it is small and something I can lift off will a piece of fine sandpaper I never mark it a problem.

 It looks like some kind of food or liquid stain. It's pretty small and I suppose I could try to sandpaper it off, but IMO, I shouldn't have to fix it. The sender should have done that. However, since I could probably get the stain off with sandpaper I guess I won't RWAP.

You are right, you shouldn't and you have every right to RWAP for a food stain. But when it comes to small stuff, my personal experience is that it usually is less of a hassle to try to take the stain off. If I can, I don't RWAP it, but I most certainly tell them exactly what you said here - that there was a stain on the book, I have been able to remove it, but it isnt the responsibility of the receiver to fix upostable problems and many won't and they would be out book, postage and credit so they should try removing those small stains with sandpaper themselves before mailing them because the site's posting rules allow for no stains at all, no matter how small.

eta: I also always wait to log the book in until I have attempted to remove the stain ad will RWAP if a can't get it off.  If it is an oil based food stain, it likely won't come off clean with sandpaper.