Without seeing the car in person does make it a little harder to put an accurate number on it. It is nicely optioned, but that is about where it ends. I believe with the amount of rust showing, is like looking at an iceberg, most of it is hidden under the water line. IF, the amount of rust showing is a true indicator, the rest of the hidden metal is the same if not worse. From looking at the rear of the car, it will need quarters, dutchman panel, trunk gutters, trunk pan, trunk extensions, rear cross member and probably a couple wheel houses, maybe both inners and outers. The metal alone will be in the 2-4k range just to purchase, then you have to figure install. And once you get that far into it, what are the frame rails gonna look like, then there is the rough structure and it just keeps going.
For the record, I am working on a 71 Gran Coupe with an "N" code motor, It has been off the road since the early to mid 90's and it is in better shape than the one pictured. I have spent a bunch on metal and just about ready to get the rebuild started. Unless you can get it a good price, this one may be deep hole project.
Others may look at it differently, but I am seeing some really good, hard to find options with a VERY rusty shell with some tags and a title, if it has one.
With all said, I am going to say its value at 12k is way high, and should be closer to half that. I would pass on it at 5k. Take the price of what the guy wants, add up all the metal and parts you will have to buy, then figure in a paint job. Now take that figure and look for a done car within that price range.