The difference is, in that situation it's just a bad trader, in the other situation the entire market has to be cleared by a central authority - the corn dealer or company management. If they turn out to be a bad trader the entire market is invalid and it doesn't matter what system[...]
If the "entire market" is a single corn dealer or one lone public company, that may be true. But that's a dumb way to set up a stock system anyway (strawman?). No one in their right minds would want a block chain for every company on NASDAQ -- over 2,000 block chains for that one market.
The bitcoin decentralized notary system would be well suited, however, to the task of transferring shares of stock for multiple companies, between untrusted parties.