I have been semi-eyeballing a decent blue Banker's Special in the used-consignment shelf at my favorite LGS/range for quite some time. It is almost at the one-year-on the shelf date. ...
Over 6 months at a local gun shop, which the public can enter and see it every day? That tells you it's too high for the local market. And the local gun shop knows that. Maybe the consignor doesn't.
With some caveats: The job of a seller is to get the absolute highest price he can. The job of a buyer is to get the lowest. All you can do is ask for a lower price to be considered. I'd go a couple hundred down. Why? Because you'd know right away if the guy is serious about selling it, of if he's happy to just "store" his gun there. If he comes halfway up, would you be happy with the price?*
I've made offers on consigned guns many times, and almost always they were accepted. It's not about "comparisons" with Gunbroker. It's about the person selling it, what his motivations are, and if he's tired of it sitting in the shop for months with no calls. Your offer is about what YOU think it's worth, using your knowledge of rebluing, problems, broken things on it.
*nobody here knows what this gun looks like or if it's original.