I know ARCs are not postable here, but I picked up one (first time ever I've had one) at our library sale and put it in my TBR list. Now I see a "W a" next to it which means it's wishlisted and set to automatic request. That must mean the actual book has the same ISBN as the ARC, right? I will have to be careful when I finish it so that I don't accidently get it posted into the system.

Yes, ARCs are often printed with the ISBN of the eventual book. It's also common for the ARC to be paperback and to be printed with the ISBN of the hardcover final release.

If it is helpful, on those unpostable types of books, I use a Sharpie and write "UP" (for unpostable) in a certain spot inside the front cover as soon as I get them.   Then I always check that spot before posting, in case there's a time lapse between receiving and reading, so I know not to post.

Jeanne, you can also add a Note to the book listing on your TBR, reminding you that your copy is an ARC - it shows up as a little yellow post-it note.  I use this feature for all kinds of notes that are visible only to me.

thanks for all the suggestions!