It's hard to tell if you're playing devil's advocate here but this argument works against your own self-interest, not the other way around.
Saying data can be used to manipulate a transaction applies to literally everything in capitalism. The same is true in the reverse; less data means insider information is harder to identify, leading to dangerously bad investment decisions when the data finally comes to light and the market inevitably corrects (whether it's a little or a lot, but in this case, let's pretend it's a lot).
Having more data helps you make an informed decision as a buyer, especially when you can look back and determine if a sale was sane and reasonable given the state of the market (demand/supply). Without more context, it's just a guessing game controlled by those who have the supply and data - how is that better?
Seems straightforward to me but idk.
Also, as Shawn says on ReservedInvestments, "Grading is always an insidejob."