To make sure I understand… you birddog for a guy and he helped you to create a direct mailing piece, which you sent to 50 people. Your contact info was on that mailing piece and one of the homeowners has now called you about selling their house.
Basically, there should be some communication on his part on what you should do once the calls start to come in (since he had you put your phone number on the letter).
He really needs to train you regarding what information he wants you to gather from the homeowner (in order for him to further determine if he wants to buy the house). Ask him if he has a script or just a list of questions that he wants you to ask homeowners when they call. In addition, ask him how he wants your to present the situation to the homeowner. He really should be giving you guidance on this.
Since he helped you to create the letter, it seems that you are working closely together on this; so don't be shy about communicating your concerns and/or questions to him directly.