Technically, eBay doesn't. Nor does the seller.
The Canadian governments and the shipper charge and get those fees.
Remember too that 'import charges' come in three parts.
Duty may not be charged if the item was manufactured outside a NAFTA country (Chile, USA, Mexico, Canada).
There is no duty on used goods.
Sales tax is charged on imports whether they are dutiable or not.
There is sales tax on used goods.
Service fees, sometimes called 'brokerage fees', are set by the carrier. GSP is ~$5. Canada Post $9.95. Other couriers like UPS often charge $25 or more.
The carrier usually * won't turn over the item until the import fees are paid.
There is duty on 'gifts' with a value over $60
$20 Cdn is about $15USD.
* FedEx is sneaky, they bill you later, after you have signed for the item which also agrees to pay the import fees.