After that, you leave a feedback as you have said so that others dealing with him would be cautious about their engagement with him via the p2p platform that they don't get scammed and if you can, block that vendor so you don't encounter him again.
Yes you are right, If he is not being truthful about it then the feedback would speak of volume about his attitude of such scheme by him. Sometimes they succeed on people with such scheme claiming to be a mistake that they really did not look into the figures to know what they are doing as a result of the fact that they just woke up from sleep and other cooked up stories from them just to cover up their tricks.
If one wants to be up and doing on the p2p transactions, they would need to do away with attitudes that would scare potential customers from them because if a negative feedback is left for left on their trade profile, it just would end up pulling away deals from them and nobody would look into their ads for any reason because they would be extremely careful and cautious of such trader.