Crypto exchanges and crypto retail shops may make a ton of profit by running fractional reserves just like banks do with fiat money, pumping out paper bitcoin to customers who don't take custody of their bitcoin.

What does it take to get big exchanges ot shops that provide instant and 100% transparency /self audit, where you see their pool of bitcoin on one or more adresses and you instantly can follow all of the inflow as well as all of the outflow?

Is this technically feasible, will it be costly, do we already have players who offer this type of service or do you have any other insights on this question?

Furthermore, what happens in practice when we get big run on multiple exchanges/retailrs and suddenly everyone want to take self custody ?

For instance, what happens in such a scenario with the transaction fees, and how long will it take to reveal if they have been running ponzis?