Are taxes supposed to be calculated depending on the seller, or the buyer? We have a client that aims to ship his product internationally. (I think right now it mostly aims to be US and Canada, but the system would be open to people from all over the world to be bought)

Thing is, I can't seem to find a clear and definitive answer: Are taxes supposed to be calculated depending on the Buyers location, or the Sellers Location?

Examples:

If Seller is in the US, and Buyer is in BC, Canada; do I have to charge the BC GST/PST (about 12% on the price)?

If Seller is in QC, Canada and buyer is in the US, do I have to charge TPS/TVQ (about 15%)?

Also, in this case, there's a physical product being shipped, this isn't a webservice or something like that.
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.