It depends.
I think it is in very bad taste to use someone's presentation of an effect without their permission. For example, I've seen people copy Jeff McBride's Miser's Dream presentation.
As for effects, if you have purchased the effect, you are free to use it anyway you want (unless certain rights such as for television or video are reserved).
I'm not a big fan of learning tricks by watching them or trying to figure them out. If someone has worked to come up with a creative method, it is up to them to decide if they will share it (by selling it or otherwise). That being said, there are a lot of effects that are pretty simple and anyone with a basic understanding of card magic will figure them out. I think it is worse to "teach" those effects without proper crediting (i.e. charging for someone else's secret) or to reveal the methods on the internet.
Let me give you an example. I know how Wayne Houchin does French Kiss, but I purchased the effect anyway (on the Art of Magic DVD) when I was at a lecture by Wayne. I felt that if I was going to use his unique presentation, that I should purchase the effect. I was glad I did because I also learned Carbon Copy which is one of my favorite effects.
It is interesting that you refer to Two Card Monte. I don't think that is the properl name of the effect you are talking about because that one involves the use of a double face card.
http://www.penguinmagic.com/p/1228
However, I suspect that you are referring to Eddie Fechter's "Be Honest, What Is It" in The Magic of Eddie Fechter by Jerry Mentzer (1974):
Or you are referring to "Switchcraft" which is attributed to Ted Annemann and John Scarne and first appears in Frank Garcia's Million Dollar Card Secrets (1972):
An based on those, there are a lot of variations of two card transpositions going back to Professor Hoffman's More Magic in 1876:
http://www.conjuringarchive.com/show.php?cat=1602