Middleman is a link between trades by a trusted user to ensure safety.
Eg, you're selling an an account, buy the buyer doesn't want to go first and neither do you.
A middleman comes along you both trust, he receives the payment from the buyer and works with you to secure the account to the buyer. once all is changed he pays you the money the buyer game him.
You can only be the one who deem if a person is trusted or not. Usually by looking through his vouches,