Stocks can be purchased and sold at the same time if you are doing arbitrage trading. So what is it exactly? Well, basically as a trader it is a way of capitalising on the differences in price of either the same stock using different trading platforms or two stocks using pair trading. These differences in prices to show up as anomalies in the market and often don’t last long and are linked to the activity of the stock. It’s these price movements that create the trading opportunities.