Maybe you can only leave trust after both parties have agreed to do a deal. It's silly being able to leave trust for anyone anytime. Can you go on ebay and leave everybody negative feedback without even trading with them?
That would be hard to do. Being able to define when someone has done a trade would have to be done manually. Which would require the staff to do a lot of work. That also requires hiring a lot more staff. Even then you have staff which will be involved in a private trade. Which a lot of people will not agree to as a lot like their privacy.
The system is also flawed because a newb can scam a load of other newbs and the trust wont even show up and he can continue scamming. A honest person can have loads of positive trades and none of them will even show up, but one "trusted" member leaves him negative for no reason and boom, he now looks like a scammer.
It's the persons responsibility to search up the person who has left negative trust. You wouldn't trust a random person on the street so why would you on the internet. You would want to ask questions and research. The trust system is a guideline. Makes it easier to contact those who left bad feedback for the particular person. Granted some people do see the trust system as a end all system. As soon as they see "Trade with caution" they immediately assume they are a scammer. But, people have been proven to be irrational and there is no fix for that. None of them will show up? You can enable this on your profile to show untrusted ratings by default.
Can you go on ebay and leave everybody negative feedback without even trading with them?
No you can't. But, this site isn't just a trading site like ebay. If someone is acting suspicious then people can state that in the trust section. Remember trading is a very small part of the forum. People mainly come here to find out information and discuss. I would say a big part of the forum too is seeing what the world thinks about Bitcoin.
I would also like to state that if you allowed only to give feedback/trust when you have traded with someone it would be blocking others giving trust for people posting malicious links, websites and other suspicious activities.