More importantly it is not up to 3rd parties to dictate to the rest of the forum what their acceptable level of risk is.
I mean, I agree with you here, but that isn't what Default Trust does. If we were banning these users and deleting their threads, then you would have a completely valid point, but I disagree that simply putting the words "Warning: Trade with extreme caution" under someone's avatar amounts to dictating to other users.
If marking people negative isn't a deterrent, then what is the point? If it is then you are punishing them frivolously. If it isn't then what is the point? What is being accomplished that could not be done with a neutral?