I'd like to think I'm an ethical person, but I'd have snapped that up in a heartbeat.
As said in other posts, the seller could have done some research, there's enough on the dial to know it's not a $5 watch. They're probably happy thinking they managed to bang out a knackered old watch for a fiver.
I think everyone has a story of when they've missed out on buying something which has gone up on value.
I missed out on a MK1 Golf GTi for £400, which in good nick would be something like £25k now. I also laughingly turned down £5k for what I now know to be a Hakosuka Skyline about 20 years ago, a nice one of those would be stupid money now, last I saw was £140k.
I know it seems like I'm money-orientated, but I'd service and keep the speedy, it's just nice to know if I needed to, it has value.