I've been running afoul of region restrictions that have prevented me from making any purchases on Steam since around March, even though all of my account details with both the credit card company and Steam reflect that I live in the US. Recently, Steam Support helped me get to the bottom of the issue.
Apparently, one of the common anti-fraud measures across the credit card industry relies on looking up the Bank Identification Number ("BIN"--first 6 digits in your credit card number) to make sure your billing address and issuing bank are in the same country.
Unfortunately, nobody in the industry is in charge of making sure the BIN lookup databases are actually correct, and of course it's possible to be stale or simply incorrect. In my case, even though the issuing bank for my card is indeed in the US, about 1/3 of the vendors I checked returned incorrect information listing the bank's location in either the UK or Malaysia.
Valve isn't responsible for the incorrect data, but the good news is, Support can still help you out.
If you are also running into region restrictions, and you think it's in error, I'd recommend you look up your BIN. If it comes back with incorrect or unexpected information, then you can mention this in your support ticket, and Steam Support can do a kind of secondary validation process. Following this I was able to buy games again.
Note this won't help you circumvent any restrictions--it will only help you clear up situations where the restrictions are being enforced in error.
Hope this helps!