I also encountered the same situation, the final fight skype told me to use probiller START CHAT solve the problem of buying gold!

It seems its "region blocked". As lame as that is, its nothing new. Just cause you are not from the US you cant do this and this. Have seen this crap all over the internet (youtube most of the times).

I have solved this issue by setting up a proxy IP in a different country. Since there is no issue with the card itself, once your IP is registered in one of the bigger countries, the purchase block is lifted.

I haven't into this problem yet but seeing that I don't live in the USA either I'll keep that info in mind.

It's mostly the banks which set up annoying location/geo-blocking.
I had issues with purchasing EU game time for other games simply because I was in NA.

My credit card by default was set that way. I had to call up the bank all the time to authorize stuff and go through all the rigmarole of proving I was the card owner every time -_-
If I recall correct I eventually just told them to disable the "protection".