A person has to be in your trust network first to be able to make an impact on another person's trust ratings.
Default user trust list have many users. Even if you don't add DefaultTrust, other people who add it can see the feedback as trusted.
If a newbie posts trust feedbacks without references, then everyone will know that it's an obvious scam.
Not all time. Trades can also be done in other places such as LBC & Bitcoin-OTC.
-MZ