I don't understand how the bid could even be accepted... Why would he be able to PM but not post in the forum. I really only see two plausible reasons.

#1. The bidder knows/isn't sure he'll be able to fulfill the bid so this way, it will be much harder for anyone to discredit him if he doesn't pay.
#2. Inside bid (no disrespect OP, it just doesn't make sense to me).