yup -- one on the back, a different one on the inside.  i am pretty sure i can post it with either (or both), and to be nice just pm the requestor a "caveat emptor" to make sure it's the one they want before accepting -- i was just so surprised to see it, i'm curious if any of you wise people knows how often it happens that a book gets two isbn's assigned?  then again, i don't even know what made me compare the numbers, so maybe it happens a lot more than any of us are aware??!?  thanks for your thoughts!

Many books have an ISBN on the back and then a different one on the inside. The one on the inside is probably for an alternate version-like the hardcover or paperback (don't know which one you have), large print or maybe it's original edition. The book should be posted by the ISBN on the back of the book.  Make sure the binding matches when you post.

LOL One of the unpostables I've already sent out had 3 ISBN's.  One on the back cover (which brought up the correct book).  One on the spine (which had 11 digits and didn't even remotely resemble the one on the back even to the first several digits).  And a third completely different one that brought up an entirely different title from the same publishing house on the inside flyleaf.  I personally thought it was lack of quality control as the book was a harlequin release.

EDIT: Try both of the ISBN's in the search If your book matches the Title, Author, Binding, and ISBN, only then would it be appropriate to post it.  You might double check that the publisher matches as well, BUT when the ISBN matches, the publishing house will also match as this is how they are distributed (this would only allow you to double check the database here more than anything else).

I loved PBS's description of where to find the ISBN number because on roughly half of the first 200 books I posted there wasn't anything on the back cover.  The directions assume that these are books that have been printed using the unwritten rule of thumbs that have come about since the 90's.

Many books published since 2007 currently have 2 IBSNs, a 10-digit (all there was before 2007) and a 13-digit, and both of those unique ISBN go together with one book (ISBNs used to be all 10-digit, but they ran out of numbers and had to expand to the larger 13-digit system).  The 13-digit ISBN is related to the 10-digit.

In my experience, the most accurate ISBN is one (when available) found near the barcode on the book (i.e. back cover of mass market paperbacks).  As Mary said, ISBNs listed on the verso of the title page are often for different versions (which has never made sense to me, but there it is).

thanks for your responses, mary, chris, and elizabeth.  in this case both isbn's are the same amount of digits and call up the right book with the right binding from the right publisher -- go figure (and if it happens often like mary said, no wonder they ran out of isbn's)!  chris, i think we should start a contest for the most isbn's on a single copy of a book -- so far you're in the lead!  ;-)