In a true market-moving demand situation, the bot's supply of bitcoin would be exhausted. A similar argument would result in exhaustion of dollars to buy with in a true market-moving supply situation. However, if the bot is colluding with the exchange, then the exchange can have effective unlimited supply/demand by 'printing money' in either mtgox usd of mtgox btc subject to avoiding a 'run-on-the-bank' and trying to balance out the accounts over time.

This scenario makes a strong argument for having exchanges operate on the bitcoin network so that at least the btc side of the exchange is subject to global auditing.

-s