IMO the guy did you wrong by all means but you should have really either took this car to a well lighted place at night and called around about the trans before hand to find out how much its worth. The problem with trading anymore is the guys dont look at what they are getting, they glance at it.
Again i feel you got hosed on the deal but I am affraid you are stuck with it. Get stuff in writing next time that stuff hold up in court.