Positive returns are ensured if you can buy low and sell high.
Arbitrage is the process of buying at a lower price in one market and selling at a higher price in another, where the arbitrager knows both prices and the price differential exceeds transaction costs.
For example, if an ounce of gold is $279 in Tokyo but $288 in London, an arbitrager can make $9 per ounce (minus transaction costs and any difference in buy/sell prices) by buying in Tokyo and selling in London.
Traders relentlessly seek riskless profits through arbitrage. When intermediaries buy in a market with a lower price, demand grows, driving up the price. When they sell in the market with the higher price, the greater supply pushes the price down. Thus, arbitrage reduces transaction costs and pushes relative prices toward equality in all markets. For example, arbitragers finesse any need for you to travel to London take advantage of better deals on gold available there. Intermediation promotes economic efficiency by linking markets that are spread geographically, so goods are moved from areas where they have a relatively low value to markets where the goods are more highly valued.