Arbitrage
The process of buying an asset (such as shares) and then immediately selling it in a different market so as to profit from the difference.
Arbitrageurs can exploit tiny differences in the quoted price of an identical instrument across different markets using very large-sized trades.
If the person buys a stock for 50 USD at one exchange, then sells it at a different one for 55 USD, he would make profit without even risking money.
Of course, such opportunities are rare as prices across exchanges are typically synchronized.