Any import valued over $20CDN (about $14USD) can be opened and assessed for applicable duty.
http://www.cbsa-asfc.gc.ca/import/menu-eng.html
At the same time the item will be assessed for Canadian sales taxes.
Some items are not dutiable (used items for example, or bullion) but are taxable (used items).
So expect to pay sales tax.
The carrier is also allowed to charge a service fee for wrangling the item through this process. Canada Post charges $9.95 and couriers $25 and up depending on the assessed value.
Yes, that means you may pay more that the purchase price of the item in import fees.
To confuse the matter, while couriers like UPS must follow the law, CBSA and Canada Post employees have come to the sensible conclusion that it makes no sense to assess items when the cost of assessment is higher than the possible fees. Do they only look at high value imports or very bulky imports.
Which means your seller should ONLY use the postal system for shipping. (And Air Mail, but that's a different set of reasons.)