TBH, based solely on that thread “Gordy” comes across to me as having a jocular writing style rather than being an arse. “At least I didn’t get ripped off” looks like cynicism regarding (any) forum trading in general rather than you in particular; in short, don’t take it personally.
I don’t think you did anything wrong in selling it elsewhere, unless that’s specifically against the Classified rules (eg, places like AVForums explicitly state ‘if you’re selling it here, don’t sell it anywhere else’). It’s probably good form to state your intentions up front if that’s what you’re doing, though, then it avoids any unpleasant misunderstandings. I also think your reaction was a bit, ah, reactionary.
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