Its a very simple system an any cryptocurrency are listed in two exchange some time one place price and another price are Change so a people buy a low and they sell these are another exchange that is buying low and sell high system.
Such differences can be seen very rarely and with very few coins this'll happen. In the past when exchanges operated within a specific region or within a country this had been there. Now what we have for trading practice is different. The Exchanges were universal and have the common price. In the past there were people who have been making good money buying it in one country and selling on the other country.
Arbitrage opportunities still appear from time to time but what happens is that they last a very short amount of time, and this is the case because there are many people that are looking for those opportunities and are using bots to do so, so no human can compete for those opportunities anymore since those price differences can disappear in seconds, and if to this we add you need a lot of capital to do something like this then it is easy to see why this is not a popular way to trade the markets among the average traders.