"Truth is a defense."
True, but the person making the allegation has the burden of proof to establish that his statement is true. If the truth of the statement is in a gray area, the person making the statement is more likely to loose. Winning or loosing a case often depends on which party has the burden of proof.
Better be sure your proof is solid and not just some third party's opinion. If you are relying on someone's first hand knowledge of what actually happened, you better get a written statement so that the story doesn't change later on.
Without some insider's confession, I don't know how you'd be able to establish (i) the identity of people that participate in an internet auction and (ii) that one of those persons was acting as a shill for the seller.
