Check out YouTube.
A few guys I follow there are Raiken Profits, Prof Sales, and RockstarFlipper. There are others. The ones I've named have been on eBay for years. They all make a pretty good income. They all have valuable content for new to experienced resellers.
There are pros and cons to selling on eBay and Amazon. First, you're on their platform so you have to obey the rules. Any mishaps and you could get thrown off. There goes your income. Second, they can change the rules at any time. You have to remember that they are in business to please the customer first. They will always have the customer in mind when they make their changes. The HUGE plus is both platforms are well established with a huge buying community. Amazon more so than eBay. But, if you do it right you can do well on eBay. It's like anything. You gotta put the time in.
Evidently you want to transition from eBay/Amazon to your own online store. The upside is it's your store. You can do what the hell you want. The downside will be no one knows you. It would take you 10 plus years to maybe get to the level of Amazon or eBay. The community is already baked in on eBay and Amazon. That's huge. It's why both platforms are so attractive to sell on.
Good luck.