The items need not be purchased in Canada, it can be something you brought in. This also applies in the United States. What customs is looking for is revenue (duty). Anything foreign made and brought into the country is subject to duty. Duty only has to be paid once. However if you take it out of the country and return with it you have to be in the position to prove duty has already been paid. The way to do this is to register it before you leave to prove you left with it.
If you never registered it and you brought it back any number of times, you are liable for duty. That is why you see items stamped made in (country named).
Every piece of camera equiptment I take out of the country is registered. I learned my lesson years ago when everything except a Kodak camera was registered. I said hey that's a Kodak. All he did was point to the made in country and said it was foreign made.