Well, I'm a Brooks fan and I have brand new ones and older, vintage Brooks, so I'll give my opinion. There is nothing structurally wrong with that saddle. The "wet" marks, indicate it was wet at least once. No big deal. Didn't hurt anything. I think the price is a touch high, even for Chicago market. Velomine will sell a new one for somewhere around $10.00 higher than that. They would have to charge shipping unless you can pick up in Springfield, IL.
Personally, I bought one almost identical but without the rash on the back, for $40.00. Looks to me like the owner had a flat tire and turned the bike upside down and the leather was rubbed raw in the back. This is merely cosmetic, but still matters.
See if the owner will take $40 or $50 and see what happens. I'm staring at a Brooks Professional right now that I put on a fixed gear bike I just bought yesterday. It was abused, has some cracks in the leather on the rear 1/3 of the saddle, but it still rides wonderfully!