I have conscience issues when I come up against something where the seller doesn't know the value, as well. If they give me a price, I'm fine with that. If they ask for an offer, it's hard. I ended up paying not much below market for a set of Campy 63 Gran Sport hubs, when I could have had them for $20. I just couldn't bring myself to do it. It was made worse by the fact that the seller was an old guy, selling them on behalf of an old lady.

I don't think you did a bad thing though - he was out to offload it, and did better for it because of you, perhaps. It's his responsibility to find out the value before he sells.