I received an email yesterday that one of my WL books was available. I went straight to my account and requested it. I got an email just a little while later saying that my request had been accepted. This morning, I had an email saying that the book cannot be mailed. Reason given by the sender: Waiting for another member's request. Can someone explain to me what this means? I'm really bummed, because I was really looking forward to reading this book.

They may have tried to post it to a friend's WL and it offered it to you instead.  This happens sometimes.  You should still be in the #1 spot. 

I am not sure why it said that.   But I made a mistake today.  I posted two books as a surprize to a friends shelf.   Forgot that MY shelf was on hold.   So I took it off hold and ooops those books were posted to someone else.   I felt badly about it but I bought those books to be a gift for my friend.  So - I had to mark that I could not mail the books.     Mistakes happen.