i'd been watching for one for a while to fill out my dad's set (he's on his own for a 28 ga) and what i saw closed on gunbroker just about any intact unmolested 20 will bring $400 at least. for long distance sale you gotta add shipping and ffl xfr fees so that $400 turns into well past $450. and this gun is where you can hold it in your hands before paying for it.
the subject of shooting 2-3/4" shells in the short chambered model 12's comes up pretty regularly here and on the other board and opinions run the gamut from "buy 2.5" shells" to "i've shot 2.75" shells in mine since 1952 and never had a problem". as jim said, it's a good arguing point for a lower price. then leave it alone anyway.
roger