I am not a lawyer - but I expect there are plenty about.
I think this is a question of contract law and the place a contract is regarded as being located. A contract consists (and I expect I am simplifying excessively) of an offer and an acceptance, and I believe it is the physical place of acceptance that determines where the contract is, and therefore under which country's laws any questions relating to that contract can be settled.
Of course a contract may contain terms overriding that default position.
So if a bookseller in Canada offers a book for sale in Canada and you go there and accept his offer, the sale takes place in Canada and will be governed by Canadian laws, including their copyright laws. You'd could then happily take the book you have bought in Canada to any other country, having only to worry about specific import restrictions rather than copyright issues.
I believe, however, that if you accept his offer by writing him a letter and posting it in (eg) the US, the sale would be governed by US law and the US copyright laws would apply. He would probably be in breach of US copyright law by sending it to you in the US.
After that I get a bit lost: I don't know where the acceptance is located if you send a fax, or an e-mail (and maybe the two are different).
But I'm sure someone better informed than me can make it all clear.