The problem with this is that it could, and definitely would be abused. People could post 'warnings' about people in the Trading Forums, thus possibly ruining their reputation for no reason. People would post things like that just to get back at someone for something, perhaps a defeat in a duel, disagreement over some topic, or just a long-standing grudge.
Also, there are many many topics/posts in the trading forums, it's much easier for a person to just check the top pages of the Scammer Accusation forum, if they want to see if there are any very recent accusations about a person. In there, you can see all the accusations bumped/posted in for the last day in just 1 and 1/2 pages, in the Trading forums it would be more easily missed. Of course, an accusation on its own doesn't prove anything, though you are right, usually a large amount of accusations in a very short period sends of warning signals to potential traders.
To add on to the above, this has been suggested before and denied.
Quote (juggalo_4_life @ Apr 16 2012 07:05pm)
well they could make it where false accusations result in warn/ban
How could it be proven that it was false then? Not every S/A case has enough proof to lock, should everyone with not enough proof be warned/banned?