This Sugya discusses the concept of "Ein adam makneh davar she'lo bah l'olam" - a person cannot sell or purchase that which has not yet come to fruition. We discuss the various reasons for this limitation and some of its ramifications.  The evident connection to Parshas Toldos is Yaakov's purchase of the birthright from Eisav.  What exactly did Yaakov buy, and how can one purchase something which does not yet exist, are questions pondered by many Rishonim and Acharonim.