A couple of years back I almost purchased a stereo amplifier from the states. Never having bought a large ticket item I decided to check out what happened at he border by calling first calling the shipping company and then calling customs canada.
If the part is made in North America there should be no duty. If it is made outside of North America then duty can be applied at the border. Taxes can be applied at either.
I don't know how this applies to used stuff?
Thats just what I got from the folks at the border. And it certainly seems to apply to items I have ordered for work.