Perhaps now is the time to discuss actual implementation of a reputation system. As it stand, a German scammer probably got away with some bitcoins.
I thik we might just make it simple : a database of evaluations of market participants. It doesn't have to be decentralised. Google will do the job. Here is how I see it.
All participants should be identified by a GPG public key. Once a trade is successful, both participants should sign and publish a message such as :
-----BEGIN PGP SIGNED MESSAGE-----
On October, 23th 2010, I made a commercial transaction with a person whose public GPG key was IDXXXXXX.
The transaction was done without any problem.
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.10 (GNU/Linux)
-----END PGP SIGNATURE-----
I'll do this kind of things on my site (http://grondilu.freeshell.org/bitXmetal.html
Google robots would collect all those informations, so that one could have a fairly large amount of report from any GPG ID key.
It wouldn't be perfect, but it would do the job not so bad I think.