Thanks for the list. Bookfinder.com is truly a great link !
I looked and found some great deals already on biblio.com
It depends what I am looking for, in my case not only English language. I have discounts and author/editor's prices for some as well. But here goes:
I use Amazon France, simply because it is often cheaper than UK when exchange rates are high and UK postage added, whereas above €25 or as a Prime customer they will be delivered free.
BookFinder will search for the cheapest source and do so across countries and languages: http://www.bookfinder.com/
Book Depository are good: http://www.bookdepository.com/
Second hand with proceeds to charity: https://gogoodbooks.com/
Rare and hard to find books: http://www.bookfinder.com/
From USA but using sales to fund charitable work: http://www.betterworldbooks.com/