Are taxes supposed to be calculated depending on the seller, or the buyer?
The most generic answer is that governments can basically tax whatever they want, according to whatever rules they choose to invent.  I have seen situations in which goods or services have been taxed by both the importing and exporting governments.

Having said that, there is a clear international norm that taxes such as VAT (the equivalent of sales tax in the vast majority of countries) apply in the importing country, after import duties (so you pay tax on the import duty and transport costs).

I don't know about the specifics of the US and Canada; I believe in particular US state sales taxes don't follow international norms.  NAFTA may also specify.