I won't get into the fairness of the price you should pay for the coin! However, if genuine, it is a collectible coin which has some value above the value of the Gold it contains. I have included some links below for help in determining whether the coin is a genuine Gold Sovereign but even these may not help as some very excellent counterfeits of these coins are out there and it is one of the most counterfeited Gold coins in the world.
I'd offer him nothing until the coin is authenticated, rated and slabbed with a clean 'bill of health'.
Otherwise, it would be like buying a car without first checking to see if it at least had an engine.
Don...
More than likely I'm not going to buy it. Because I want to keep it and I need to resell it. So maybe another day. I'm not sure they want to sell it either so I was just trying to find out a value for them on it.
The last thing I would do with a gold sovereign is slab it. Talk about a big waste of money. These are very common, even that old. Slabs are the biggest waste of money. I collect colonials and first thing most colonial coin collectors do when encountering a slab is to break the coin out and throw the plastic away. Slabs are great for the crooks that tell you some common Morgan dollar in MS 67 is worth 2 or 3 or 10 times what an MS 66 is. Slabs are a bunch of crap. Kind of worth as much as a JD Power & Associates rating in my opinion, nothing.
You can see if it is gold by weighing it and checking the weight against the standard. I would not pay extra for Rare date based on that auction. It seems to be based on a mint mark. Either way, They are cool coins.