The term "Master Lease" has multiple meanings to real estate lawyers. Sometimes it describes the underlying, primary lease in the subleasing context. In other circumstances, it describes a seller's guaranty of certain "rent" payments for a period of time after closing with respect to space in a building that is sold to a third party. Based on your question, I believe you are referring to another use of the term in the context of seller financing of a property. Here is a link to an article that summarizes the pros and cons:
http://www.reiclub.com/realestateblog/2009/02/23/who-needs-banks-when-we-have-the-master-lease/ Hope that helps.
Mar 17, 2009