That is a nice shotgun in pretty nice condition considering its' age. I don't see a lot of vintage German doubles that have a straight grip stock. It certainly doesn't look like it has ever been refinished. Some here would say that the mirror bores would indicate they have been honed, since it was built prior to the introduction of non-corrosive priming. But I'd say it was simply proof that the owner was a guy who knew enough to clean and oil the bores after shooting it.
Cleaning the flats with some 0000 steel wool and oil would not hurt the value. I would refrain from using any abrasive cloth or sandpaper at all. And if you get exuberant and start polishing the rest of the gun, all bets are off. Incorrect cleaning and polishing, no matter how well your intentions, will adversely affect the value. The outside of the frame looks like it still has a fair amount of case hardening colors hidden under some oxidation. Some guys here have found that case hardening colors in this condition can be dramatically restored with cleaning in an ultrasonic cleaner. You can do a search of old threads here for ultrasonic cleaning, and probably see photos of what is possible. So don't scrub that stuff off.
There is also a sub-forum here that specializes in German and Austrian guns, and you might get some more information from guys who hang out there. The best way I know of to ascertain value is to watch the prices for completed sales on the larger gun auction sites. But you have to compare guns in the same grade and overall condition, and remember that even things like the time of year or the day and time an auction closes can affect the final sales price. Opinions of value are often misleading at best.
