The HC says a PM is required and the condition must be agreed upon before the book is sent.  What happens if you never hear back from the person requesting the book?  I have already accepted the request, and sent the PM. If I never hear back, I won't be able to click that the book has been sent. If there is more than one copy in the system, I'll drop to the bottom, right?  If  I cancel the request, I'll also drop back to the bottom, correct?

Is there any way to keep from being dropped to the end of the FIFO list?

If you don't hear back and your the transaction is cancelled,  I would think you could contact the Admin Team (blue "contact us" link at the bottom of any page) -- they really encourage communication, and if the other person is not communicating back, they'll check into it.  You could also ask them if there is any way you could be placed back at the head of the line.