As I understand it, if there are multiple books in the system and someone requests that book, then the oldest book is sent out.  If the person with the oldest book turns down the request to send the book, for example if they can't meet a condition of the book being in a pet free house, then the next oldest book will be sent. 

So what happens to the oldest book?  Does it remain first in line for the next time that title is requested, or does it drop to the bottom of the line? 

If, at the stage of theRC, it is marked as does not meet requester conditions, then the book will go back to being the first in line. If the order is cancelled or marked Cannot Mail, then the book will just be removed from the poster's shelf. They will have to re-post it and start at the end of the line again.

Also if you put a book you have on your shelf on hold-it'll move it to the end of the line when you unhold the book.  It only holds your place in line if you put your whole account on hold. 

If the requestor cancels the transaction then the oldest book goes back at the top of the FIFO line.