I received a book that has a few wrinkley pages, like 5 or 6, that seem like it may be warping from water.  It's hard to tell.

The pages don't stick together at all, and they're entirely readable, but I don't want to end up crediting someone for a book I won't be able to repost.

When marking books received, I always err on the side of caution:

If I'm not sure/can't tell then I give the benefit of the doubt. Maybe the sender couldn't tell either, or simply failed to notice it altogether.

But, If it looks like a water stain, then it's a water stain. I will mark the transaction as "problem" and also not repost it.

It's up to you whether you want to ask for your credit back. We all occasionally get "borderline" books. 

If there is no staining, it may simply be publisher's wavy pages. Unfortunately they are using very cheap paper nowadays and many perfectly good books have wavy pages.

Thanks this helped a lot.  I flipped the pages, and see there is a stain along the edge, so it's definitely water damage.

Hopefully the sender will positively resolve the situation and issue your credit back. I always find that if I include a photo of the damage when I PM, I get a better response from the sender.

If it's a WL book, you can always try the Book Bazaar and ask for a credit. If someone who is wishing for it wants it like, YESTERDAY, they may be willing to pay a credit for it.

With the cheaper paper and glue that publishers have been using lately, it seems a lot more people are complaining about getting books with wavy pages. In the discussions here, several people have talked about that cheap paper doing that from exposure to high humidity-- so if you leave a book on the porch, or in the car on a rainy day, or out by the pool you may be creating an unpostable.

I've gotten a few of those, where it's a little bit wavy or just a few pages. The book is perfectly readable, so I keep it and read it but then don't post it again, just to be safe. It's a pain, but I did get to read it, so I'm good. As PBS says, sometimes you get a book that has one more read in it. It's better to take the iffy ones out of the system and save everyone the same concerns.