I've noticed lately that a lot of folks seem to have copies of books on their WL that are useless. Some books get put into the system without an ISBN, and are assigned a "made-up" one.  But once that copy is requested, that number will never be used again, so having it on your WL will never help you get it.

So if you see a book with a less-than-10-digit ISBN, don't add it to your WL! 

Actually, as much as it's unlikely, the PBS "ISBN" listings can be used again.  I had a book club edition that I sent off and when the person was done reading it they used the same listing I had created to repost the book.  I've had enough WL offers that were non-ISBN listings to warrant giving it a shot.   When it comes to looking for an edition that was published before the ISBN era or a book club edition there's really no other way to indicate that you're looking for that particular book.  And while it may take up space on the Wish List, if you've got the room, it can on occasion be worth it.

I re-use them IF the information match such that I feel certain it's the same book. If I have a paperback copy of "My Wife the Penguin" published by Rediculous Books in 1965, and there's a system number for that title/author/year/binding I don't re-invent the wheel.

Hmm, I always thought they weren't supposed to be re-used, but this makes sense.  Sorry!