This sounds like the OP bought a stolen bike. I'm betting that he will be unable to contact the seller at this point.

It seems doubtful to me that the seller "forgot" about the pitlock on the seatpost. If I were selling a bike, I would remove them and put in basic hex screws again...those pitlocks aren't cheap. If I were selling them with the bike, it would be mentioned as a feature for an added premium, with the key included.

I smell a rat.