I just received a book that has pink stains along the edges and into the margins of the pages.  There is a very tiny bit of warping on the first few pages and the cover.  The book is usuable, readable etc but I believe the pink stains are probably Kool-aide or something (ie water damage) So I dont view the book as postable.  I have no intention of asking for a credit back as I need the book for research and was not intending to relist it however should it still be marked RWAP, should I just send a polite note or should I not worry about it?

If it has liquid stains, and the liquid caused warping, it sounds like it's probably RWAP.

I understand that you want the book, and that you're not intending to post it again. But many people do want to repost books after reading them, so people need to be discouraged from sending unpostable books. :-(

I think I would mark it, but send a polite message saying you don't need the credit back and just wanted them to be aware of the problem.

I've been on the receiving end of water-damaged books twice. The first time I asked for (and finally got) credit from the sender. The second time, I just reported the book as RWAP and sent the sender a polite note to let them know that the book is not repostable. It's a little frustrating that some members don't realize that we're trading two things here: books, and the potential for trading the received book in the future.