The model 12, as it came to be known, is actually the model 1912 (which gives you a clear indication of when it was first released). It was and is the hammerless evolution of Winchester’s fabled model 1897 (or ‘97). The model 12 was designed for nitro powders and are mostly chambered in 2 3/4”.
The model was designed before the era of removable chokes, so they are almost exclusively fixed chokes. This, alone, has been the greatest factor in suppressing prices for this model. While the steel used in manufacturing model 12s is rugged, use of steel shot through the fixed (and most often “full”) chokes is ill advised (although having the chokes opened up is possible). Having the barrel threaded for removable chokes is typically not economical.
However, the model 12 still has a niche in the uplands, where steel shot is not mandated. It is also still viable as a target gun, where lead shot also remains prevalent.
I’m not up to speed on the less common variants of the model 12, of which yours clearly is one. Your photos do not allow us to see much of the gun, but enough to have an idea of what we’re looking at. The vent rib, the raised comb, the adjustable butt plate, and the contoured pistol grip are all hallmarks of a dedicated trap model. This much is certain, you bought the right configuration for trap.
Millions of model 12s were produced. And, with being mostly unsuitable for migratory bird hunting, as well as being commonly available, they historically have not commanded great prices. Depending on the configuration and condition, they typically sold for between $300 and $800. In the past 3 years though, prices for distinctive configurations and sub-gauge models have been firming up and starting to increase. IMO, the price range for these is currently $400 to $1200. Your specific model 12 (without better photos and more detail) looks to be found at the upper end of the range - say $700 to $1000.
BTW, if your employee didn’t mention it, this is a takedown model. That is to say, the receiver/stock can be separated from the barrel/magazine assembly for transportation, storage, and cleaning. It is cleverly designed so the takedown mechanism can be easily adjusted should looseness develop in the joint.