“Catallactics is the praxeological theory of the way the free market system reaches ratios and prices.
It aims to analyse all actions based on monetary calculation and trace the formation of prices back to the point where an agent makes his or her choices. It explains prices as they are and not as they should be. The laws of catallactics are not value judgements, but aim to be exact, objective and of universal validity“. http://en.wikipedia.org/wiki/Catallactics
You can read the chapter on Catallactics in Human Action by following the link. http://mises.org/humanaction/chap14sec1.asp
This is another impressive video in Praxgirl‘s series.
The next lesson, The Market https://hessenflow.wordpress.com/2012/02/14/praxeology-the-market/