Colby Price (the creepy real-estate agent played by Canadian actor William B. Davis), his purpose in the movie, if any, was never explained, he just appeared for no reason at random times. One theory is that he was a ghost and that John murdered him along with his own family when the farm failed. This has yet to be confirmed.
The trivia item below may give away important plot points.
It's mentioned above that the Banker that offered to buy back the farm had no real purpose, in Burwell's flashbacks after the crow attack you can see it is him that hands Burwell the reposetion/foreclosure notice. That is what set the wheels in motion for Burwell's wife to attempt to leave him leading to his killing of his family. What isn't explained is where Burwell has been for the 6 years since the murders and why he has no recollection prior to the crows attacking him.