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.
Based on checking his feedback, he hasn't sold any email forwarding since he was removed from DT2. That confirms your theory as of why people bought it.
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.
That isn't completely true. I can transfer a domain name to you without any information at all other than your registrar account name.
I wouldn't call a domain naim a physical good. Those are things you can touch after they arrive at your real shipping address.