Let me explain the difference to you.It's very different from selling XXX games in the US.
You can advertise an XXX game in the US. There are no laws forbidding that. You can put the box out on an open shelf in full view of everyone that comes in, provided there is no XXX material on the cover. The only thing US law requires is age verification of the customer.
Many stores don't carry or don't display such things as a matter of corporate policy, but that's not law.
You can't advertise unrated games in Germany. However, you can buy and sell as much as you like, if you are at least 18 years old.
It's the same kind of corporate policy that keeps such games off the German Ebay branch and many stores. In fact it's only an MS corporate decision to not launch Gears of War in Germany, because they absolutely could if they wanted to.
However, I'll grant you that the organization doing all the rating is pretty fucked up, full of old geezers who lack understanding of games, not to mention respect towards games as a first-class form of entertainment and art. Might be worse than in the US. At least they aren't so afraid of nipples over here.