Actually, Tim, it is my understanding that if you call German Rail (at that number), they can sell, over the phone, any tickets or reservations that you can buy from a DB counter in Germany. I've never tried this personally, but I believe some people have reported that they have actually done this. If anyone has successfully done this, or tried it and it didn't work, please let us know.
I'm guessing that they have to manually access another data base to book tickets outside of Germany, and that is why there online system can mostly only sell only tickets for trains with one terminus in Germany.
I know of two exceptions to this "rule":
Connecting trains in Switzerland. For instance, you can purchase online a ticket from Munich to Interlaken with a change of trains in Zurich. From Basel, you can purchase online a ticket to Interlaken from the Basel Bad Bahnhof, which is actually in Switzerland but considered a DB "border station", but you cannot purchase online a ticket from the SBB (Swiss Rail) Bahnhof in Basel.
City Night Line (CNL) is a subsidiary of German Rail. They run night trains through Germany. A few CNL trains have both termini outside Germany (e.g. Amsterdam to Zürich or Vienna). You can buy tickets with reservations online for these trains.
Note: DB used to sell reservations only (passholder fare or supplement for accommodations, called Aufpreise) online for night trains, but I don't see them anymore.