Well, "German system" is really a bit of a misnomer. There were a number of 'systems' that stemmed from the original "six key" clarinet of the virtuoso who started the "modern" clarinet.
Once the six key horn came about, things started to diverge. The six key horn was progressively developed by both French/Belgian and German makers. The F/B group put together something that ended up being called the "Albert" system.
Roughly at the same time, a Frenchman named Klose developed the "Boehm" system horn, the one that is the standard in about 90% of the world today. It got the Boehm name because it used the Boehm feature of "ring" keywork (called 'Brille' in German) to operate 'keys' (these are the actual cups and pads covering the holes, not the touchpieces that operate them) situated to eliminate several of the cross fingerings that were still needed on the six key horn. (Boehm originally used these on the flute, where he made his name in music.)
"Ah, but I notice that there are some rings on these horns, yet they are not Boehm system," I hear you say. Well, that's because the Albert system also incorporated them as time when on. However, they still used the odd first finger system (odd, that is, to what the folks accustomed to the Boehm fingerings).
Meanwhile, ve Chermans were not idle. Makers in Germany took the same, based on the six key horn, clarinet and continued the development of instrument towards what is now the standard on that side of the Rhine. The final polishing of the design was completed by one Oskar Oehler (who died during the Second World War as a Luftwaffe musician), which is now known as the Oehler system instrument.
Oehler instruments are easily picked out of the crowd of "near German" Albert horns by one feature, a plate for the right hand middle finger. Iffen it doesn't have that plate in the place of a tone hole, then you don't have the modern definition of a professional clarinet. It may be a subspecies of some deviation from the German norm of today, but it ain't a "German" clarinet under current standards.
Which is better, Albert, Oehler or Boehm. Well, I own (and used to play) all three, so I view myself as someone with more experience than most other players. And, I always say up front that most tend to discount the Albert horns, as there are some tendencies to play a bit sharp on them. (There is also the finger spread on the lower joints, which some find hard to deal with - on an A Albert horn, it would be much more of a bother.)
As for the preference between Oehler and Boehm -- well, that's something you developed when you learned to play one or the other. The Oehler improvements were made to deal with the intonation issues only partially addressed by the Albert system, and the design does a good job of accomplishing that daunting task. It did so with more touchpieces, more tone holes, and some peculiar adjustment techniques to make it all work right.
(My Oehler, a Czech built instrument with silver plating (and a horrid silver logo placed just about everywhere) had rings that were set too high for my slim, fat-free fingers to seal against the chimney below them. Setting ring height on a Boehm horn is a pretty simple matter. Doing so on a full blown Oehler is something else entirely, as there are a couple of vent keys attached to the rings themselves. In order to adjust the rings, you had to fit custom cork pads to the vent keys, carefully filing down the cork until it was thin enough to allow for the ring height adjustment, and yet still seal the tone holes. It took my technician (the now deceased Fred, a find of a lifetime and he was sorely missed when he left the building) about a week of spare time puttering around to do this, first by moving the ring height, then carefully designing a set of spacers that did the sealing, and then using the spacers as templates to make the final pads. It was worth the effort, and I hope that my final Christmas gift to Fred made his remaining time that much more enjoyable.)
As the Oehler operates along the lines of the Albert (with the same odd (to the Boehm player) first finger treatment on each hand, and no thumb ring or thumb ring seat to stabilize your grip on the back of the horns), it takes some getting used to, but once you have it under your fingers, it works well enough.
The one great negative about the Oehler horns is that the little finger work is a bit more complex than one the Boehm. You don't have the same degree of duplication of the little finger keys that you have on a standard Boehm instrument. The so-called "patent C#" mechanism makes up for this to some extent, but you still don't have the same facility on the Oehler (or the Albert) that you do on the Boehm. This, and the funky first finger arrangement are enough to put most players not raised on them off of the Oehlers and Alberts.
That said, there are plenty of spectacular Oehler players out there in the motherland. And, there used to be as many on the Albert system. The famed Lazarus methods were put together by the English player of that name, who played the Albert system to the day of his death. This accounts for a few of the exercises in the method, which seem like a doddle on the Boehm for those who play them, yet they serve very real purposes on the Albert, with its less enabled lower joint. And, as I said, jazz players seem to like them.
Finally, I learned to play on an Albert bass clarinet pitched in A, a relic of my grandfather's post-World War I experience, when he and his lovely wife made it over here from Bavaria, running from the Red revolutionaries, with only their portable valuables. Grandma pawned her jewelry, and Grandpa pawned three of his four Albert system clarinets. The fourth, a wonderful Buffet bass pitched in A, the pawn shop would not take as there was no established price for such a rara avis. It went in a series of closets until it was time for little Terry to abandon the string world (what the hell were they thinking?) and start playing the clarinet.
My mother, as cheap as the day was long (she was of Prussian blood, not at all like my Bavarian father's family), decided that we already had a clarinet, and there you go. I had three or four lessons from a teacher who ultimately gave up on the Albert system. But, the Rubank method had an Albert fingering chart (it's the one that looks like a real clarinet - the metal one was the Boehm), and so I plugged away and here I am. So, it can be done.
So, who's your potential market? Some jazz players swear by the Albert. (Woody Allen, a halfway decent jazz clarinetist, actually had Selmer make him a new one when his beloved Albert horn suffered a terminal crack.) But, Woody's not in the market any longer, so you'll have one less potential customer. Alberts do not "move well" on eBay, and I'd not be investing much money in any of them unless they are in pristine condition.
If, by some chance of fate, my memory failed me and these are, indeed, German horns, then your market is only a smidgen less limited. I have a bonafide German clarinet of the Oehler system, and I used to play it regularly when I did Broadway shows, as it has more facility in the sharp keys that seem to populate that style of music. (For "normal" work, it was Boehm all the way.) Dealing with the system differences is daunting at first, but once you get into the groove, you can get along about as well as you can on the other horns. (We do, after all, switch from clarinet to oboe to saxophone to bassoon and get by there, right?)
That said, if you are up in years, you won't be as flexible with the change. We don't learn as well once we age. So, many older players will decide not to try something different. Another market wedge set aside.
For German horns, the place to advertise would be ebay.de (or whatever the German abbreviation is on the internet these days). That's where you'll find the biggest concentration of buyers for German clarinets.
And, there are instrument collectors out there. Recall the famous saxophone collection that is up on eBay for some fantastic amount. Most don't want a saxophone pitched in F, but those folks are out there - connect with one who's into getting every variety of clarinet horn that's out there and you may make a sale.