I listed a book today, but the ISBN brought up a different book. It wasn't a pb/hb issue, it was a different author and different book. I listed mine without an ISBN and put a note in it to remind myself. If the book is ordered, should I just leave the note in it to give the person a heads up if they try to relist it?

They'll know there's a problem with the ISBN just for the fact that you posted it using a PBS assigned ISBN.  You can leave the note in there if you want.  

Well...my memory isn't that good, lol, and I guess I'm odd in that I don't put books I receive on my shelf, so if it were me, I wouldn't remember that a book I ordered had a PBS ISBN, especially if one were present. I think I'll just leave the note in it in case it goes to another "odd" person.

The book I wanted to post had no ISBN number but I found the same book already listed. In Amazon if such a book were already listed I could just click sell (or list) another or maybe find the ISBN with the listed book.

When I asked I was just told that I had to run through the no ISBN procedure with the second part of the answer not making any sense.

Dear John,

This is a quick note to tell you that you have a new personal message at
PaperBackSwap. This new message is from Patti I.  (craftypatti).

----------------[Message]---------------

SUBJECT: RE: Live Help Question

If the book does not have an ISBN you do need to go through the process of
posting a book without the ISBN.  Sorry.  That way it is the correct
book/edition, etc.
 

Look on the copyright page.  The ISBN is often listed there on the older books.  Or books that have lost their dust jackets.

Of course that does not help with reissued ISBNs such as Jodi's.  Or when one number was applied to a whole series of books.  There is a publisher of classics and children's classics that the whole set got one single number. 

ETA.  Live Help is staffed by VOLUNTEER tour guides.  Not TPTB.  But basically us.  Members who volunteer.

Plus, ISBNs only go back to 1970  (SBN -9 digit - 1966) so anything older HAS to be listed as A Book Without an ISBN.   Publishers MOSTLY re-use ISBNs for different editions of the same book - hardcover, paperback, large print, even audio.  But some older ones for books that weren't very popular are re-used for entirely different books.  A real problem for PBS.  sigh...

One publisher sold off a whole section of it's company.  Then went on to reuse all of the old numbers.  So there are a number of old Sci Fi paperbacks that have the same numbers as a line of series romances.

I was listing some textbooks recently from Pearson and McDougal Littell and found that almost every book had the same ISBN as an older book. It was a real pain having to list each of them as a book without an ISBN (but many of them got requested within a few hours of my posting them, so I guess people were able to find them anyway).