I can see a couple of problems with that. If the forum gets involved in moderating scams then when someone does get scammed they are going to blame the forum moderators for letting it happen. Conflict of interest might also crop up if a moderator is involved in doing business on the forum they could get accused of shutting out the competition.
Obviously disclosure would be needed to avoid that.
I'm also not referring to all potential scam threads but more so to the few with the tale tale signs. Another thought would be to tag the thread with a warning until the OP provides necessary proof.
However, threads started by noobs, advertising goods/services whilst not accepting escrow and preventing comments by locking/self-moderating are the ones I'm referring to more specifically.