I live in Canada and so far I have had no problems buying books via B&N even with my Canadian ip. I just have my credit card shipping and billing address listed at a US address on my B&N account. If you had to you could use a proxy to show your address as in the US.
If you did want somebody else to buy books for you, they could email them to you and you then could add them to your Nook via your computer. As long as the relative used your account that your Nook is registered under you should have no problem adding them. I imagine magazines would be the same, but I have never used one so I am not sure.
I do not think you should have a problem buying books though. There is a thread that is at the top of this sub-forum for international Nook users and lots of people there download books all the time.