arbitrage

Only large institutional investors and hedge funds are capable of taking advantage of arbitrage opportunities. Because they're able to trade large blocks of shares, they can pocket millions in arbitrage profits even if the spread between two security prices is small (and it usually is just pennies).

By contrast, individual investors typically don't have the large sums of money needed to take advantage of arbitrage opportunities, and trading fees would eat up any profits an individual arbitrageur hoped to secure. Institutional investors aren't burdened by these same limitations.

Of course, small investors and entrepreneurs take advantage of much smaller arbitrage opportunities every single day.  For example, if you've ever purchased a bargain-priced item at a garage sale or flea market, and then sold that item for a higher price on eBay, then you've profited from a form of arbitrage.

The main creator of arbitrage opportunity used to be a lack of real-time communication about prices in other markets, but modern technology has reduced the number of arbitrage opportunities out there. The relatively few arbitrage opportunities that do exist are elusive and don't last for long -- when people realize that a security is cheaper in one market than another, their interest in exploiting the opportunity will drive up the price of the "cheap" security and drive down the price of the "expensive" security until there is no longer a price difference. In this manner, arbitrage does a good job of ensuring equilibrium in the markets.