You want transactions to be easily reversible?

Payment for order: what data was sold (or do you mean the orders as data)?

You mean broker as in a pure broker, not broker-dealer?

What about policing algorithms, for example? Who'd do that and how?

Who provides liquidity? Just natural supply and demand?

https://en.wikipedia.org/wiki/Payment_for_order_flow

Sorry, I thought you just had some idea about basic financial services.