Arbitrage. The idea is simple, but doing it is risky too.

A friendly reminder - Don't be swayed right away by exchange prices. Please do a thorough research before buying because aside from a possibility of low trading volumes mentioned by @semobo, sometimes, exchanges that have above and/or below average market price are having problems like Wallet Maintenance. You might buy and expect that it can be sold at a higher price, but in the end you'll find out you won't be able to send it to a specific exchange.