That statement implies that a contract requires something to be "written." Someone's "word" or actions in many cases have legally constituted a contract. Besides that, you have stated that the details were enumerated on this site through a private message, which makes them "written." You've only shared the side of the story which makes it appear that the seller has done something wrong, when it may just be something you don't agree with and then called for public outcry over his behavior.
It's also clear that his original post he intended to sell it for $500 more than what the shop would offer him. Quoting him out of that context is not an objective representation of the situation.
If you're going to make those implications in public it's only fair that all of the relevant information be presented, especially when you as the accuser are in possession of that information.
I am not in favor of either party, only the way it has been represented and the way information appears to have been withheld.