Amanda, if it's a surface smudge some members have success using a fine grit sandpaper to remove markings from page edges.

Otherwise, if the requester does not make contact you will need to cancel the transaction.  Then pat yourself on the back for catching the unpostable before it becomes a RWAP.  So sorry about the postage, but it's probably cheaper than refunding a credit.

After seeing your pic, I would definitely  cancel the transaction. It's not worth a RWAP. LIke Denise, said losing the money you spent for postage is cheaper than losing a credit.

When this has happened to me before, I simply cancel the transaction and send the book on anyway and hope the person can still enjoy it.   I usually include a little note or PM the person explaining cancelling since it was unpostable but sending it as a courtesy.

Sorry you need to cancel the transaction.Explain what happened to the requestor. 
Most PBS postage has to be used on the date the label is printed (or when you stated it would be sent when printing).

Just a good reminder to check the book before clicking that you can send it. I've had various things happen too and had to cancel. (old glue breaking when checking the book before sending )

Melissa when you cancel (unless it's a WL book) it automatically reorders the book for them. So when you send yours anyway-they might just end up with 2 copies and an unpostable they don't know what to do with. 

When I notice a book is unpostable after I've accepted, I just cancel.  If I have time, I might check their WL to see if I have a WL book in my TBR that I can read quickly. Then I might PM and offer to post the WL book them and send the unpostable book as a freebie. But usually I just cancel and then use the damaged book in the Damaged Books Virtual Box or post it on Bookmooch. 

After looking at the picture, I'd say most definitly cancel.

I've had that happen a couple times before and offered to send the book as a freebie once and the other time invited the person a 3/1 deal on my shelf since at that point I had a lot of books that were going pretty quickly.

Yep, time to cancel. I probably would have just canceled when I first noticed the stain.

What I have decided to do in this situation is to PM the sender saying I've found a problem and am therefore going to cancel. I describe the problem and say that they can PM me if they still want the book. But I don't wait for their response; I go ahead and cancel the transaction. If they still want the book, I can send it to them - but outside the normal system because we're not supposed to post unpostable books. That means, of course, that I have no recourse if they don't give me a credit when they receive the book. So far, when someone still wanted the book, I've received the credit, but naturally I'm accepting a risk.

Melissa's approach is even nicer, of course - assuming that they want the book at all in that condition, which wouldn't have been the case with one of the books that I recently realized I shouldn't have ever posted. Probably I should switch to her approach, since I'm rarely hurting for credits. Of course, with a WL book that would probably mean that the person would get two copies, which would be more of a nuisance than a help, so I guess I should be careful with that. A PM would probably still be appropriate to figure out what would be best.

I had something similar happen to me last week. The person has requestor conditions.  In double checking the book I noticed a barely visible stain about the size of a nickel on the bottom of one of the pages.  It was not even obscuring the text but I canceled anyway.  I  didn't want to take a chance on a RWAP.  Lucky for me I havent't even printed a label yet so I was out no money.

As others have said, I would cancel. I had that happen once too and from that lesson learned never to accept a request until I'd checked the book over again. (I check them thoroughly before ever posting them, but sometimes books are on my shelf for months/years before being requested, so for me it's just good practice to double check again.) As long as it's you cancelling the request, the request would be passed along to the next person in FIFO with the book, or if it's wishlisted, the requestor gets reinstated to the front of the WL queue.

Cheryl