No idea about the logistics, or what the investigation found out... so I can't give you a full answer, but I can give you the events from my side in a certain situation.
I accepted the friend request of some guy I healed or something (that will be slightly important in a bit). After that he whispers me asking how much I want for Dersu. I get him to clarify because it was a bit vague. So he clearly asks if I would sell the account, and for how much.
I sent a report ticket about the character.
Next day I am logged on a low level alt, and this same guy whispers asking if I want to buy a level 45 Ranger. I tell him how unlucky he is, and report him a second time.
Within a few hours I was in a support tool back-and-forth with support answering some questions about the characters involved.
Anyway, I saw the character one more time (the third day) and never again. ... this is why the friend list was important.
So, either he was banned for attempting to buy and sell with me... or they found through an investigation that he actually HAD bought/sold with someone else.