I'm not disagreeing with you as such, it's just that a newbie in this business has a better chance of succeeding with a niche store than a general store. You were either very lucky, or else you have a very good grasp of eCommerce and marketing. Most of the people on this forum have no idea, and have a better chance of succeeding if they go the niche way.
When I was a kid, I tried to sell things and failed dismally - subscriptions of newspapers door to door, that sort of thing. I finally decided that I just wasn't a salesman.
Then when I left school, I bumped into an old friend who worked in a toyshop. I used to go in there to chat with him, and also to check out the toys - there were awesome toys in the store, like building kits etcetera, that I really liked. Then one day the store got really busy - it was just before christmas and people were there to buy presents for their kids. My friend was alone and couldn't cope, so I started helping out by answering questions that customers had. I made a whole bunch of sales. Then I realised that I COULD actually sell stuff, if I knew the niche and was excited by it. That's why it's good for beginners in this business to choose a niche they are passionate about, themselves.
Now, after years of experience, I can sell anything, but first I need to understand the niche, and know what makes its potential customers tick.