I ordered two VERY common books with more than 100 copies in the system (at least). I ordered on Dec. 29, and my tab kept saying that the system was waiting for a response from the member. Today, I see that the books have been mailed, but they were mailed by different senders and in my Transaction File were the two requests that "timed out" I think because there was no response from the sender. Is there some way I would have been informed of that other than looking at my Transaction File? IIRC, the timed out request just goes to the next sender, but I was surprised to look at my Transaction File and see two big red X's

That's the only way you'd know.  No other notice is given.

The request will keep automatically rolling over from sender to sender until one accepts and mails.  Those timed out ones just float to your transaction archive and stay there.

Also if you watch your "requested books" tab, and note the date and time that you requested the book, then you can go back and check each day (or every two days) to see if that date has changed.  If it has then it's timed out and gone to the next one in the FIFO, and will continue to do that until someone accepts your request.  Pat