Well I'm sorry. With the exception of the warranty on the engine, you bought the vehicle "as is". Does that sound better? If you buy a used vehicle and there is paperwork, the paperwork spells out the conditions of the deal.
I'm assuming you saw and test drove the vehicle in question? Did the vehicle have license plates on it when you saw it? If there were, you should have noticed. Even if there weren't license plates on it, I still think you're going to have an uphill battle. If there were no plates on it, you could TRY and argue that he was concealing the fact that the registration had expired. Again, it's going to be an uphill battle.
How long ago did you purchase this vehicle? Have you brought it to the attention of the seller? If you have, what did the seller say?