Some of us buy quite a bit OS, usually small stuff like eyepieces, mounting plates and dovetails, guiderings etc. Postage isn't too bad when small and light and if its under $1000 there's no customs duty. For large items like mounts you don't really save that much. I've just bought a Losmandy G11 from OPT in the states. The price is great until you factor in postage, customs, GST, etc. I didn't really save much in the long run. Also if something goes wrong its expensive to return it to the supplier for warranty repairs. The same reasoning applies to complete systems and OTAs.
So the short of it is, small light items under $1000
Large, bulky expensive items
Many of the US based astronomy stores are very good to deal with. If you do a search I know there was a couple of threads about this very topic.
My favorite OS store - Optcorp.