How does one become DT1? I always assumed they were hand-picked by theymos, which would put him at 100% by definition (which isn't necessarily a bad thing, just stating the obvious here).
Theymos is in charge of adding people to DT1.
I've noticed before that many of the dark green trusted people are very active trading physical items, and earned most of their trust in those sections. That's not necessarily a bad thing, but it is something that can quite easily be farmed: buy some items from the right people now, and a year later you're at +80 or more!
People obviously do just do trades with the right people to get trusted feedback. The easiest to abuse though are people selling digital goods for fairly cheap and that is what was happening with TheButterZone's emails he was selling. The vast majority - if not all - the people who purchased them had likely just worked out they could buy a cheap 'green feedback' from him. Buying physical goods is much more expensive and you have to give out personal information that exposes you which is something I wouldn't want to do here for obvious reasons.