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.
I don't think it would be that hard to implement, nor does it have to involve staff. Just both users have to agree to a trade by checking a box or clicking a link etc before any feedback can be left.