If you are the seller, shipping to the USA, this is not your problem. The buyer is responsible to know if he will pay duty and sales taxes. The shipper (whether USPS or a courier) will also add a brokerage charge.
If you are the buyer, you are responsible for import charges on any item valued over $20 CDN (~$15USD) .
To find the duty (if any) start here:
http://www.cbsa-asfc.gc.ca/import/postal-postale/menu-eng.html
Used items are not dutiable.
All items are taxable and the rate will depend on your province.
Canada Post will charge a $9.95 service fee. Couriers' service fees start around $25. You will not be given the parcel until you pay.
If your seller is using the Global Shipping Program, and this is the type of item you would want to buy under the GSP, you will see the exact amount of 'import fees' on the listing BEFORE you agree to pay. If you don't like the total, don't click the second time to complete the payment.
