How many people do this? Is it a grey area?
I admit that I've been doing it for a long time, and I encourage my friends/relatives to do so as the prices you can get are MUCH cheaper than anything on the NZ marketplace.
I had a chat with Microsoft support about it, and they said that it was OK but in certain countries it's possible it could cause a few problems.
Some people I've talked to scream that you'll get banned, but Microsoft support told me they haven't had any cases were a user got banned for this.
I don't have an Xbone, but I did this for a while with Steam, before I "moved" to Malaysia, and now enjoy Ringgit pricing on everything :)