You're making assumptions about things SE knows, like who is and who is not a gilseller. They have already stated they don't even know for sure who is a bot and can't ban them until they are sure. (And gilsellers don't just get their money from bots, some get it the same way legit players do, like playing the AH). Also I would think if they were positive someone is a gilseller they would ban them immediately rather than let them run around transferring gil to other players.
And it's ridiculous to think SE would spend money to have this "judge and jury" you're talking about. Since it's not an actual crime, SE would have to hire people and pay them to spend all their time reviewing everyone's gil history and all the people they've had gil transferred to or from, in a game with 4 million active accounts.
