Precisely. I'm not entirely sure how deep your site goes, but assuming it's like the actual stock market, one could potentially game the system by creating two

Precisely. I'm not entirely sure how deep your site goes, but assuming it's like the actual stock market, one could potentially game the system by creating two separate accounts and making a series of investments with one, and by shorting those exact same investments on the other; One of the accounts is guaranteed to go up.