From experience, B&N will only let you buy ebooks from them (using a gift card or foreign credit card) if:
(a) you have a (bogus) US mailing address attached to your account; and
(b) you have an IP which B&N recognises as being based in the US.
(a) is easy to do, but (b) is a bit harder. I live in Australia, and use a VPN (I use StrongVPN (
www.strongvpn.com), which for US$55/yr gives me a US IP) when purchasing.
There may be free VPN solutions around, but I've never had a problem with StrongVPN. Also, call me crazy, but I like to think that the fact that I am paying for the VPN service means that crediit card details etc. are less likely to be hacked than if I'm using a free service.
Best of luck!