<mods, maybe this ought to be moved to the Canada forum section?>
What red said... Here's what I generally do:
1) If I believe there is no chance of selling it into the US due to my buy-in cost vs what people south have reported, I price for the Canadian market. So I scour the recently sold and current listings to see what the going rate is for US sellers. Also, I look at what are they charging to ship to Canada and/or are they using the GSP which makes most items over $25 just not worth it (to the educated buyer) if they can buy in Canada. And I price accordingly... usually it means I'm about 15% higher than the US sellers but I'm hoping to sell in Canada anyways.
2) For items where I've had a competitive buy in price, I just accept that I'm making a little less since everyone takes a piece of the pie. For example, I found a dozen Rovers in July at a local mom & pop toy store. My price on them was $39.58 per - I just sold one for $115. US shipped last week to a US buyer. Sure I make less than someone in the US but after exchange, shipping and fees, I took home $55. I'm happy.
3) I usually list in US funds - lately the exchange rate has been in our favour. A gamble I take as I really do believe the US buyers are more sensitive to seeing nice round numbers... Canadians are so used to shopping in the US for deals, I think you can squeeze a few extra bucks out of Canadian buyers this way.
4) ALWAYS print your labels through PayPal for parcels, and NEVER for padded envelopes! You get 10-15% off by printing through them for parcels. PayPal's envelope rate always treats it as if it's a padded envelope that won't fit through the envelope slot. Now they'll often try to get you to pay the full rate at the post office but I personally insist on shipping it as a regular mail item which usually costs between $2 and $4. It's about $12 to send a padded envelope as a parcel which is the same rate as a small box. (unless of course the contents are expensive enough that you can't afford to lose then I'd do it the parcel way as it's trackable)
And... just for good measure, I always try to sell stuff locally on Kijiji as well. I've had decent luck, even had a number of out of province buyers find my listings. Always a risk, but I've had no bad experiences meeting in a local parking lot (for expensive stuff), or at my home or mailing things out (invoicing via paypal).
Good luck!