KY Jon,
Simply because a gun is listed on the internet doesn't mean it is still for sale-case in point, the R13 in 28 gauge. Since there (were) only two available, not of the same grade, now only one, it isn't really going to count as comparison shopping, either.
The R13 has been gone (sold) a while. I don't know why the ads stay up for so long.
That gun is a good example of differing levels of quality in Darnes. The serial number put it in 1950s production, and it had no importers name on it. Compare the wood on it to just about anything imported with the name "Firearms Center" on it. Much of the James Wayne stuff has wood the color of a stick of butter. The engraving was sparse, exactly how an R13 should be, but, well executed.
The V19 started at $11,500, came down to $7500, and is now listed at $6500. It has a non-factory pad installed, something that seems to make many guns just a little harder to sell. It probably fits a lot more people now with the pad installed.
The last confirmed price I have for the sale of a 28 gauge V19 was last summer, a like new 70s production gun that went for $6500. I never saw the gun, but, I know who bought it-he is not a dummy, and I'll bet the gun was worth every penny. The last 28 gauge V19 I imported was just a few bucks short of $10,000, with a leather case.
I had a well known gun writer (not Larry) send me a short email a few years past asking what a widows V19 was worth. I emailed back I couldn't tell him without more information, which, seemed to annoy him greatly. But, there are just too many variables on Darnes. I wouldn't attempt to place a value on a model 12 without seeing it, why does someone think you could do that with a Darne? The first thing I want to know is what is the name of the importer, if there is one, on the gun. That gives you a start, and then you can begin to appraise quality of engraving, build, and wood. A Darne that was sold with extras, like the Bretell Darne, a case, or a second set of barrels is a pretty rare find. For some reason, nobody ever seemed to keep the box or the directions that came with them, either. People buying a second hand gun like that stuff, and will usually pay more money for it.

Best,
Ted