If two versions of a book have the same ISBN - say the trade paperback and the mass market paperback - and you can't tell the difference in the PBS system - so you order a copy and PM the sender that you only want the trade copy and to let you know if their copy is mass market so you can cancel it - and the sender agrees but then sends you the mass market version anyway - can you mark it wrong book received even if the ISBN is valid?

That's a good question.  I think you could if you had a RC that spelled this out, but if the agreement was made via PM, I'm not so sure about that.  Technically, if the ISBN and binding matched when they posted the book, they did not do anything wrong, however since you had a PM conversation about this, that might change things.  I would check with a Tour Guide on this one- this seems to be a sort of gray area.  In the future, I would put a RC that states that you only want the trade version for this book, and then turn on the RC only when ordering that book.

That's a tough one. I would say it is a communcation problem not a problem transaction. It seems the sender did not understand what you wanted in your PM. IMO, any restrictions on requests should be in RCs, not just PMed. But that of course is just my big fat opinion. Marking it RWP-wrong book certainly doesn't guarantee a credit back, the sender can still say no.

eta: after reading the help docs, I would say no you should not mark it RWP:

A "wrong book" at PBS means a book that does not match its listing in the PBS Database in one or more of the FOUR necessary aspects:  ISBN, Title, Author and Booktype. 

It matches what is in the PBS database so it is not "wrong book" by PBS standands.

I think you just have to count this swap as a learning experience.

If it were me I would not mark it RWAP.  The sender posted correctly with the right ISBN.  I know you tried to clarify things in a PM, but I think the book versions can be a difficult distinction.  I never even heard of the terms trade paperback and mass market paperback until recently on PBS.  Unless a person had 2 versions of the same book side by side for a size comparizon, or were given specific measurements to go by, it would be hard for them to know what the requestor means.

I would just chalk this disappointing trade to "that's life".

It would make book trading less complicated if ISBN's were different for each version.