Comment by reverendsteveii

It's manipulation by definition. These hidden trades exist to prevent other people from being able to act in their own best interest based on available information. This isn't about what's going on in your neighbor's house. Your neighbor's house is private, markets are public. This is about being at a public auction in the middle of bidding on something when someone else walks up to the crier, whispers a few words in his ear, and then the crier just quietly says "bidding is closed" and you have no idea who bought the item or for what price but now you have to try to price similar items.