I just bought 2 books used to post (okay, I admit, I bought more than that, but this thread only concerns two).  They have the same binding, are from the same publishing company, etc. 

The first (0867307927) shows up as a hardback in the system and the second (0867308389) shows as a paperback.  So one of them needs to be posted as no ISBN. 

The problem is, I don't know which is an accurate description of the binding.  They have a flexible posterboard like cover.  Its thicker than paperbacks (even trade size), but it isn't like the usual hardback either.  Does anyone know what I'm talking about, and can you tell me what binding these have?  I'm waiting to post them until I know. TIA.

Edited for typos...

I'd say more PB than HB

I think they are called Turtlebacks. They should probably be posted as paperbacks....I could be wrong :-)

I think Cari is correct;....they're probably Turtlebacks.

I got a Turtleback in the mail today - I was wondering what it'd be like - I like it!!!!!    It's an OLD book but has held up well with that cover!

I checked the Library of Congress Catalog.   Did you mean that you had two copies of the same book with different ISBNs or you are unsure which description of the bindings was correct because they have the same bindings?  When in doubt, I always go with LOC's catalog description (as follows).

The first ISBN only indicated that it was printed on alk. paper.  By default it should be a hardcover since the binding is not given.  The title of the book is YOUR BATHROOM.

The second ISBN showed that it was a paperback printed on alk. paper.  The title of the book is YOUR LIVING ROOM.

I meant I'm unsure which description of the bindings was correct because they have the same bindings.  So, I'm guessing since the one binding was given as paperback and they are the same, that is how I'll be posting them. 

And I skimed the previous turtleback thread, but I never associated that with these.  I suppose that it is likely these are turtlebacks ,though, since these have a spiral binding inside of the posterboard cover.  Did others with turtleback post them as paperback?

Whatever you do, don't post them as a hardback!

To be safe, I'd  post both with the no ISBN option & list "other" binding.

The book I ordered was listed as a turtleback.     I was looking for a particular book by an author and the turtleback one came up along with others. 

Here is a description I found that might apply to your book:

• A "turtleback" is one of those paperback-turned-hardbacks that one often sees in libraries, usually with the paperback color cover laminated onto the front. One is now seeing this term on used-book websites. [It turns out that "Turtleback" is the name of one of the three companies that do this sort of rebinding; the other two are Permabound and Bound-to-Stay-Bound.]

I'm wondering, if your's truly fits the tutleback description. I definitely have some paperbacks that were purchased from the FOL and they are now truly, knock on hard with your knuckles hardcover, not flexible. And that's what I'm thinking is meant by a turtleback. It sounds like yours is just made of something different than the usually cover. I have a cookbook that is more of a heavy laminated softcover. In which case, personally I think it qualifies for being called a softcover. Most people's opinion of a hardcover is, as I said, a "knock on with your knuckles and it resists" type hardness, not something that flexes.

I would list the one that shows up as PB to that listing. But the one that shows up as HC, I would list as a non-ISBN indicating it as a PB in my listing.