As for being paid to do magic, someone said it...you're being paid to entertain. Letting a cocky spectator see the difference between a card trick and magic entertainment is not inappropriate...
WRONG!!! The key phrase is "to entertain
." You are being paid to entertain. You are NOT paid to embarrass
There are some people who just can't be entertained. You know that, I'm sure. If a spectator is embarrassed about something, it would be nothing that I have done. If they embarrass themselves, then they are likely to do it one way or another. Allowing them to do a card trick is not going to hurt anything. In fact, the other spectators around them may be entertained and have been entertained more just by allowing this. Once again, the situation is rare (at least for me it is). I can't remember the last time this has happened. But the next time it does, I will probably allow it again. One exception of this is kids. I would not let a kid handle my cards. Too many other things to worry about then such as bending cards, dropping cards, etc.
Oh yeah...I forgot to mention. When I do let someone show me a trick, I always turn it into MY trick by following it up with something that fits whatever it was they were doing. For example, if they did some kind of pick a card routine, I would follow it up with an ACR using what ever card was chosen.