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If it only gives you the option of Posting The Book then there are no copies of that ISBN in the system. Then you have to add it to your WL if you want to get in line for a copy. If it says Order This Book then you can do like the others said and put it on your Reminder List to see how many copies are posted.
Don't let a large # discourage you from posting a book. Some of those books are very popular and the FIFO line moves pretty quickly.