Big 12s have become a tough sell here in the states-IF they are Spanish or American. Pity the fool with a run of the mill 12 gauge model 21 for sale these days. I don't believe that is the case with English guns that fit the narrow pigeon gun/waterfowling gun description, and this one certainly does.
jOe's price is suspiciously close to what is asked for a new Benelli Super Black Eagle, a gun he owns and fondles a lot, but, doesn't talk much about, here. When you compare tangerines to tangerines, something built for a specific purpose, that you are going to use for a specific purpose, will always cost a bit more than a standard implement. That gun is built like a tank, I'd expect good, long service out of it, service that would chew up typical English boxlocks, built in typical game gun configuration.
I'd venture these guys are a little low on price, because they are only thinking in terms of resale, not long term specific use. I doubt you would find an English gun, configured as this one is, for less than $2500 on these shores, and they would be snapped up at that price, I'm guessing.
Good Luck, with whatever you decide to do, and I'd further advise against anything that says "Benelli" on the side-spotty reputation, you know.

Best,
Ted