I love to lurk in the forums, reading other member's comments. Sometimes I find that I have a lot in common with some of the members so I will check out their bookshelves. My question is: If I order a book from one of them do I get the book from the member's bookshelf that I am checking out or do I get it from a different member who happened to post the book earlier? I am only asking because some members offer deals or unpostables.

If you do not navigate away from that member's bookshelf, you will order from that member.  In other words, make sure you hit "order this book" while looking at that member's shelf.

and don't order from the description page!  this will put you in the FIFO line...if you go to look at the description of the book go back to the member's bookshelf and order from that page