E-commerce is one of the most exciting growth industries around. According to forecasts from market researcher eMarketer, global retail e-commerce sales will more than double between 2015 and 2020, increasing from $1.7 trillion to $3.6 trillion. For investors looking to capitalize on this massive opportunity, companies such as Amazon (NASDAQ: AMZN), eBay (NASDAQ: EBAY), and MercadoLibre (NASDAQ: MELI) are attractive candidates to consider.