Key is the initial research. Finding a niche that people are buying and then niche down to a specialty for which you become the go-to person.
As an example, rather than running a general store for surf boards - one of thousands - build one for specific components of surf boards. Surfers are passionate about their sport - and boards - and will research in-depth the best specialty parts. If you can provide them and really know what the best options are, they will come to you and recommend you to everyone they know.
One of the best known eCommerce platforms is Shopify and this course will teach you how to set up a winning store and do all the above research from the start.