[3np]

No, you can’t. Given a transaction on the btc blockchain, how would you determine if actually changed hands (as opposed to internal/self-transfer), and if so, at what valuation?

Trades on sidechains or on centralized exchanges also don’t register.

[wmf]

The exchanges do produce a "tape" that is analyzed by various people but you're basically taking their word that it's not fraudulent.

[3np]

Right.

> There's a public ledger

Nothing to do with this.

> we can find out exactly how many people bought it at every price.

None of them afaik will tell you how many people. Trade data tends to be anonymous and only give you price, amount, and side for each trade. There’s no way to differentiate between 2 people trading back and forth 1 million times vs 2 million people trading once each.