PBA: The Gemara says you can’t trust a Kusi’s Kashrus because they don’t hold of Lifnei Iveir (it’s somewhere on an Amud Aleph in Chullin in the first 20 Blatt; maybe it’s 4a or so?).
The question is obvious, though. But they still hold you can’t eat Treif. So why don’t they hold that they can’t trust each other? I think Pashut P’shat is that they held that you don’t have to be Choshesh that someone is lying, even if they don’t hold of Lifnei Iveir. Therefore, when they tell you food is Kosher, you can believe them. The buyer didn’t do anything wrong because he was allowed to believe the seller. The seller didn’t do anything wrong because Lifnei Iveir doesn’t exist according to him.