You have a history of being in a party with a friend, being in the same clan, chatting with each other, Likes, previous trades. This is not hard to detect.
Further, the characters that are RMT selling are generally not players and as a result do not have achievements, do not run dungeons, do not have high level characters. These should be big red flags when it comes to trading items that are higher level. Why would an account with only a level 50 character have a Manamana?