I only use Amazon (ca) for my pre-order, I prefer to buy when the dvd is release and I always add some other stuff to reach the 39$ free shipping limit. But if I want something who is not new, I'll use marketplace (mostly Caiman) since the price is much lower than the Amazon one. Just like Sebastien I've no problem with the fix shipping price since no taxes are paid, I always buy new even if the used price is sometimes much lower (not always the case, one crazy seller always ask 4 or 5 times the Amazon price for his used dvd).
I don't buy from French Canadian website at all : too pricy and, more important, I don't trust them...
Just try Marketplace Eric, you will appreciate it. The only risk is that you could buy to many dvd
P.S : If you want I can give you a list of sellers that I have made good transaction with.
Edit : Talking of pre-order for the next few weeks I will use the "What'ya got?" topic like a crazy guy. 32 movies on pre-order are suppose to have make their way to my home at the end of june... and my familly already think that I'm insane