You are trying to protect morons from their own idiocy. Anyone who uses a complete newb for escrow without even reviewing his trust, is GUARANTEED to be robbed, it is just a matter of time. You can't protect people from their own idiocy. By going around patrolling you give these newbs the impression that some one filters out scammers here when in reality you just pick a handful of sacrificial lambs and mark them and pretend it is progress. you aren't stopping anything, you are at most delaying the inevitable. Even worse, there is NO WAY TO KNOW if these people you are simply guessing about had ill intent.
Of course people should be warned about a scammer or a potential scammer. If someone thinks that someone is a scammer then others should be warned and if they are not a scammer they can build up a good trading reputation over time and possible get the negative rating removed or downgraded to a neural.
It is obviously impossible to know without a reasonable doubt that someone is a scammer, and if this burden of proof is required in order to leave a negative warning to warn others then a scammer is going to have no problems scamming multiple people multiple times.
To say that you should wait until someone actually scams to leave a negative rating is like telling the National Weather Service to wait until
after a tornado a tornado hits to say there is a high risk of tornados. Instead the NWS warns people about when there are high chances of severe weather in a particular area so people in that area can take precautions to protect their lives and their property. They sometimes get it wrong, however the overall affect is that people are additionally safe.
If someone is warned about a potential scam they would not otherwise know about, they might realize how naive they are and decide to do additional research regarding what precautions should be taken when dealing with others.