The seller has the right to sell to whoever, and however, they want, in leiu of any formal requirements set forth by this site.
He can choose first IM, first email, or first post, or first one that makes him giggle. I dont even feel he has to post how he will choose who gets it first.... however polite that might be to do so.
There is NO DEAL until both sides confirm.
That's how I see it.
Kind of like an internet handshake..... but then, I am old fashioned.