http://www.cbsa-asfc.gc.ca/import/postal-postale/menu-eng.html
If your items are under $20 CDN (about $16 USD) there will be no duty.
If your items are over $20 CDN there may be duty and there WILL be sales taxes (from 5% to 17% depending on your province). There will also be a service fee /customs brokerage fee charged by the shipping company.
If your seller uses USPS/CanadaPost to ship, there is a good chance that the CSDA will not charge the applicable duty and tax. Or they might.
If your seller uses the GSP you WILL be charged the applicable duty and tax plus a $5 or so service fee.
You can recognize GSP sellers because the listing will include these words:
Customs services and international tracking provided
This will not appear if you order from a Gallery View Search or from a mobile device, so be careful when you reach the payment stage. If you do not want to use the GSP, do not pay. You will not get the item if you do not pay, of course, so....
The duty, sales tax and service fee do not go to the seller and it is possible that she is unaware of these charges. Nor, as an exporter, is she required to be.
