I bought a couple books from an online used bookstore. They were supposed to be paperback, but they were actually hardcovers though I didn't care at the time. Now, I'm trying to post them. The ISBN is for the paperback version and, as far as I can find, there is no record of these particular books ever being released in hardcover format. They're stamped as "ex-property of such-and-such a library", and I can't put my finger on exactly why, but the covers themselves appear to homemade.

So, are these library-bound books?

Library binding means that hard covers have mylar covering- a plastic covering for protection. They're other markings like barcode or lettering and any other markings a library uses.