MacBook Pro, Mac OS X (10.6.8)
And installing the Mac OS X 10.6.8 Update Combo reinstalls the App Store for you which may help this issue.
Restart your Mac after the combo is installed. Then try the App Store.
It's ok to do this even though you are already running v10.6.8.
same when I travel.
Your credit or debit card credentials must be associated with the same country where you reside in order to purchase apps from the App Store or media from the iTunes Store as well.
Although you can browse the iTunes Store in any country without being signed in, you can only purchase content from the iTunes Store for your own country. This is enforced via the billing address associated with your credit card or other payment method that you use with the iTunes Store, rather than your actual geographic location.
The same policy applies to the Mac App Store.