Marketing. Since there are a lot of products that are the same, it's hard to convince customers what to buy.

The toughest challenge is to acquire customers. With tools like Shopify or BigCommerce, it's fairly easy to launch an eCommerce. It could be done in a day. However, building reputation and visibility is a task of a different level of complexity.

Marketing is probably most critical part of this. In general, e-commerce has a lot of more things to worry about than marketing: product searching, sourcing, inventory, storage and fulfillment. These things all add up and take away your time, especially if you start out as a single person army. Solutions that save you from the chores would be great, so you can focus on marketing.