I can buy things from any of their sites. However...
- Amazon.co.uk refuses to ship a number of items outside of Great Britain.
- Amazon.com has expensive shipping (obviously), and now this thing.
- Amazon.de doesn't usually help. Besides, I can't read German.
Looking at the Grand Theft Auto example, it's available from Amazon.com but not Amazon.co.uk; the latter, as mentioned, refuses to ship it to me. Amazon.com... might not; I can't tell, now can I? It refuses to let me add to cart without having a Prime membership.
On amazon.co.uk, you can frequently find a different seller that ships to your country though. I often get the message "this item can't be shipped to Switzerland". Then I click on "new and used from..." and there is the same item only one pence more expensive, but I can get it shipped to CH. No idea why Amazon can't be bothered to display the one I actually can order to CH first.
So this amounts to saying that they don't want my money. Very well; there are other options.