@OP, How the user is protected , if the some user decided to scam for very first time?
In that situation its not so easy for a new user to commit scams. I beleive no one going to make deals with a newbie user who haven't got enough experience & reputation yet.
I believe you misunderstood @r1s2g3 statement, let me clear you. What he's trying to say is, what if the user trying to scam is the first scam attend could be a legendary user or any rank, that's to say the user doesn't have a warning red tag on his account how then is the guest user protected if he can see trust score.
In conclusion, he's saying we shouldn't depend on the trust system but take matters into our own hands and protect our funds by doing some research before trading with any user on forum.
That's why DYOR or Do Your Own Research is used as a phrase to caution everyone even when hiring for an escrow, as after those escrow scams that shook the whole forum community, everyone here needs to be extra cautious about conducting any such trades here or anywhere. The fact is, we don't really know the intentions of the opposite party as and when we want to trade with them, so it's better to go for trusted ones only and not choose even less trusted over them.