I know as a newbie interested in customizing your trust list it's kind of difficult if you haven't traded with any user to trust their judgement or familiar with the active scammer busters
From previous threads on this topic, I understand that a custom trust list does not comprise of people who you trust in trades, but members whose judgement you trust and recognize their ability to make important decisions without bias or prejudice.
For members you trust you can leave a positive feedback.
You're correct that's why I included the highlighted part like also you can list members you trade with and trust their judgement on others.
@DdmrDdmr, I believe this list from how I understand it was made to keep the trust system accurate. You are free to list any member you trust on your custom trust list but as the introduction message says, you should add at least 3 members form the list to keep your trust system accurate.