the security six is a good revolver but I don't think a new one was over $250. I don't think I would pay $275 for a used one. maybe on the order of $200 to $225 at most.
The Security Six is a great gun but he's asking to much for it. In very good condition it's worth about $225.00. If it's just light surface rust that can be polished off fairly easily and you want it badly enough i wouldn't go more than $225.00.
Bluebook lists the stainless Security Six: $300(100%), $255(98%), and $230(95%). As you describe it, my bet is, it is no better than 95%. Added: Book says it was discontinued in 1985. The last MSRP was $338.00.