If someone doesn't want people duplicating their cards to sell then teaching a class in how to make them seems counter-intuitive to me.
I've taught lots of different types of craft classes including card-making and scrapbooking. Once I provide the instructions my control over what is done with them is over.
"If a person who indulges in gluttony is a glutton, and a person who commits a felony is a felon, then God is an iron.”
― Spider Robinson