I'm not sure I would have revealed the secret then and there.
I think it might have been better to say, "Book me for your event and I'll have a word with you beforehand."
I suppose they might not have paid up if he'd done that, but it sends out the message that you can't just throw money and expect to be told.
I think one of the things that distinguishes exposure from teaching is the reason behind it. If the person is paying money just because he is curious, then revealing the secret is not too good. If they are buying a (say) DVD because they want to learn, that's OK.
Now, there's nothing to stop someone who is merely curious from ordering from a magic shop, or visiting one. However, that still takes a bit of effort. You must research what you want to find out about, track down a place that sells the effect, buy it, read the instructions and understand it. None of those apply to someone who says, "I'll give you £100 if you tell me how you did that."
However, there really is no hard-and-fast rule and I am not going to get preachy about it.
I am reminded of the story (no idea if it's true) allegedly about George Bernard Shaw and a young lady. I might get the precise words, or even amounts wrong, but the idea is the same.
GBS "Would you sleep with me for £10,000?"
YL "Yes."
GBS "Would you sleep with me for £10?"
YL "Of course not! What kind of girl do you think I am?"
GBS "My dear, we have already established what kind of girl you are; now we are merely trying to set the price."
