I was under the same impression, that used goods were not subject to duty, but I was challenged on that a while back.
And it looks like I was wrong (and you too, Dennis).
http://www.cbsa-asfc.gc.ca/publications/dm-md/d13/d13-10-1-eng.html
As I read this, at least some used goods are subject to duty. I tried reading it although it is written in a language I no longer speak (bureaucratic legalese) but as far as I can understand, most of the exceptions deal with previously owned industrial machinery.
However, the point is the Global Shipping Program is a Seller Protection policy that carries few benefits for buyers. Nor was it ever meant to.
It should be avoided as much as possible.
If you MUST have the item listed, contact the seller BEFORE bidding, and ask if a new private listing using USPS shipping, which includes Confirmation of Delivery, the big selling point of the GSP, would be considered.