I just check a few things, in terms of ebay.
-If there's a given -specified- Canadian shipping rate, that's a plus.
-If I can contact the seller for a Canadian shipping rate, that's a plus.
-If the rate is what I've come to accept as reasonable shipping to BC ($8-$15), that's a plus.
Just gotta feel them out.
As for reasons why people -don't- ship outside of their country, I suspect the customs process is simply too daunting/time-consuming for some to consider.