* You will need to know as much (or more) about running a business, as you know about electronics.
* If you are starting out, find products that will sell. Something everyone wants.
* Consider marketing on the internet. Popular markets include Ebay, etc. You'll need to know how to design attractive webpage layouts. Or hire someone to do it.
* If you wish to compete with existing mail order houses, it will help if you don't follow their example of tacking on those outrageous shipping charges for a low-cost lightweight order.
There is a guy who sells capacitors on the internet. Low prices. Free shipping first-class. I don't know how he does it. There could be benefit in learning the secrets of the business model of this guy.