So... Do you have anything against Quickseller? The man is doing his best to protect people here from possible scams so you can't blame him.
You have already said it.
prove them scammer
I personally don't like people who leaves negative trust fastly without any warning but I think he has a reason.
You can't prove someone scammer if they are not, he is not picking random people from forum and prove them scammer
he is just helping the forum clean by spoting the scammer and marking them, legit users have no problem with him but only people who think about scam hate him.
so better then blaming someone , try to make this place a safe heaven for buyers/sellers/traders, be honest