Okay... a sender messaged me and said she received notice from the USPS that her book had been lost and all that remains is the label. I guess she's not going to contact TPTB and just let it go lost, holding my credit up for a little extra while. My question is this, Even though she used Instant Credit, I will eventually get my credit back, right? I am hoping I am not missing this book being offered too many times while I am sitting here waiting for her book that won't come. I am assuming once this gets declared lost, I will get my spot back.... hope so, there's over 1000 wishing for this book!

Yes, you will still get your credit back when it goes Lost. 

She is supposed to contact the Team so they can cancel the transaction.  Does she say in the PM specifically that she's not going to do so?  Or does she not realize she should?  This Help Center item has the info:

USPS lost the book I sent! (they sent me a notice/the empty wrapping)

If the book is close to going Lost I woudl probably just wait it out, but if there is a long time left, I'd contact the Team myself.  They do want to hear from the sender, so it might not help you, but you can at least let them know that she's refusing to contact them (*if* that is, in fact, the case). 

Good Luck!