As I know, it is not forbidden to trade with Bitcointalk account. You can tag them, but I think it is neutral feedback is appropriate in this case, if the account has not been used for any fraud. although that won't mean much either, because it's a new account and you don't know which Legendary acc is was in the trade option if he was at all. He is scammed, and never got that one acc.
if there is no fraud, neutral is okay just as a notice for future reference.
Thank you for your response. Yes, You are right that the victim account is new and they don't know much about this. ( again sometimes they do it despite know about bitcointalk account trade is not trustworthy due to they think they are very greedy. Unfortunately at the end they victim of scam.
btw I will wait for some few opinion. If I don't get I will give him a neutral feedback.