Question is rather clear but can PBS members do this?????????

No, as when they post a book, they have to check the box that states it meets the conditions, is not an advanced reader copy, etc.

The only reason to message is if the book would meet the text book or cookbook exception and that requires the other member to agree to it.

why is it un postable?

Do you mean they posted it while knowing it was unpostable? Or discovered it was unpostable after they posted it?

The first is not allowed. They need to offer the book in the Book Bazaar if they want a credit for it. Do not decline the offer as the book will then be removed from your Wish List and you will lose your place in line. The right way to get rid of the book offer is for you to accept it and then THEY cancel mailing it. That way the offer won't get passed to the next person in line. 

The second ... well, it happens.

No way!  If it is unpostable, then that's what it means -- Do Not Post!  PBS does not cut any slack on this - and in fact, there are several places in the Help Center that explain books cannot be posted and then have qualifiers added that would make them unpostable.  It's a quick way to get embroiled in problems - and the swap of one book just isn't worth it.

I try to post one or two WL books each month, and last month posted the one that came up in line on my WL'd books shelf.  It was requested right away, but when I pulled it to wrap, I realized that it was beginning to separate from the cover, plus there was about a 1/3" separation between a couple of pages in the same area, so I cancelled the swap (which requires an explanation) and also sent a PM to the member.  They thanked me for checking before mailing.  There are usually good ways to pass along an unpostable book, unless it has water damage, or mildew -- then the 'good way' is the recycle bin! 

I was granted a book from my WL but then received an email telling me that the book had been damaged  but not too bad. Did I still want it? I replied "no" that I only want to receive books that I can post after reading them. She then cancelled the book so I stayed at my place online. I get this same thing happening from time to time and I just wanted to be sure that I was correct in my thoughts that this isn't allowed here on PBS. Now I will tell them it is a no-no. 

That's disappointing - I hope someone else posts your WL book real soon!