Overseas Online Bookshops
I have personally bought books from at least eight
overseas online bookshops. These days I mostly use Amazon or Amazon UK (there
isn't yet an Amazon Australia).
International postage charges can be quite steep so for many books local bookshops are cheaper. The UK-based shops tend to be cheaper than US-based ones for UK books; sometimes they also stock different titles.
Prices can vary considerably between shops. The price comparison sites I use most are AddAll, Bookfinder, and FetchBookInfo.