The Global Shipping Program is run by Pitney Bowes. on behalf of eBay.
https://en.wikipedia.org/wiki/Pitney_Bowes
Import charges are calculated by PB based on Canadian duty on any import valued over $20CND (~$16USD).
Assessed duty is paid to the Canadian federal government.
http://www.cbsa-asfc.gc.ca/import/postal-postale/menu-eng.html
and provincial and federal taxes (PST/HST/GSP) are charged on imports that are over $20 CDN.
Yes, there are sales taxes on used goods.
These are remitted to the federal goverment which then distributes the taxes to the appropriate province.
PB also adds a service charge (estimated to be ~$5USD) on each shipment that is assessed and charged. Some shipments are too low value and are not assessed or charged. These do not seem to attract the service charge either.
Confusing the matter, CBSA and Canada Post sensibly decided that it was silly to spend expensive labour in assessing and charging import fees when the amount to be remitted would be less than the cost of the assessment.
Couriers like PB, UPS, and FedEx do not have that luxury. With the exception of PB, couriers charge a service fee (often called customs brokerage fee) of $25 and up for wrangling shipments through customs.
You may also want to research 'hub and spoke ' distribution systems.
