Buy Books Online?

I'm looking to buy some books online, more specifically some Harry Potter books and I've checked what I think are the major online Australian stores (Bookworld, QBD, Booktopia, Angus & Robertson) along with other major shops (Amazon and Ebay). I am willing to pay for them overseas but only if I can pay for three books with $10 shipping slapped on top or five books with $20 shipping. Of course if it's a little more expensive I will live because I really want these editions of the books.

I would like to find the Philosopher's Stone/Sorcerer's Stone Adult edition (picture here)
I found this on Amazon but the new edition books started at $500 and I swear my eyes popped out of my sockets. All the used editions were worn and torn beyond repair.

I'm also looking for the Signature edition of books 1-3 and 5-7 (picture here)
I've already found the "Goblet of Fire" on another site so I will be buying that when I found time to come online again. Hopefully when I go shopping tomorrow I might be able to find some, I just wish we had more bookshops around where I lived I'd get out of the house with my kid more to go there.

If any of you have seen these around while you've been online, could you please let me know either by replying to this blog or sending me a message, I'd be forever grateful for your service.

P.S. Should I try Barnes & Nobles? That thought just occurred to me but I've heard it's hella expensive if you're overseas.