[google-serf]

The problem is that while overdraft is very profitable for the bank; you acquire a legal liability to pay back that debt.

Overdraft cannot be offered in a non fractional exchange and the exchange can't find you if you take more than your balance. Money leaving the exchange must be reconciled with holdings or they are insolvent.

What we may see is blockchain denoted exchange holdings; your exchange wallet is unique and publically verifiable (only the exchange holds the keys). You could verify with balance with the address and all trades are performed on blockchain. Bitcoin would be inappropriate for this with its long confirm times but a newer faster scrypt or primecoin variant might do it. Exciting times!