I think this might be an effective metric when used with a mutually agreed upon transaction process in order to make sure people can not leave feedback without the other user agreeing to trade first.
Which I would find unfortunate.
As it is, the trust system is not used too much, it's used too little.
People should be encouraged to leave trust whenever they feel like it, not only after they've completed a deal with someone.
We all remember at least one occasion where any trust system based solely upon earlier trades would have utterly failed: pirateat40.
That's what we should avoid by all means.
If any user does not trust any other user, the trust system should give him the means to express this mistrust, no matter what.
In an open system, this will also mean retaliation ratings.
As we can clearly see from users like Tomatocage, retaliation does not necessarily pose a problem.
1. Most scams are performed by low-level users, or by "suspicious" high-level users.
2. Most scams are detected early on by high-level users.
3. Scam warnings are needed the most by low-level users.
Any trust system should reflect this.