I ordered a book,  it took a LONG time for the book to be marked mailed, and now it hasn't arrived.  the sender has asked if it arrived,because it should have, but it hasn't.  What happens if it NEVER arrives?  How do I know they even sent it?

I don't know if it is possible to know if they really mailed it or not, but if you never recieve it the credit you used to order it with will be returned to you.  You would then have to order the book again through the system and wait to get it from the next person.