Hi, I've driven an Amazon / eBay business to $4.5 million in annual sales. The short answer is - "No, it's not easy", if it was, everybody would be doing 6 digits revenues on Amazon.
Here is an example for one of the obstacles:
It's important to understand that there is a glass ceiling to what you can do on Amazon (selling someone else's brands), if your'e doing well, Amazon will quickly notice you and start stocking and selling the product line themselves. They control all the verticals: Logistics, payments, SEO and have a tremendous buying power.
One of the possible ways to overcome the above is to create and market your own brand (OEM manufacturing being one of the possibilities), in this scenario - if your'e doing well enough, Amazon will actually approach you to purchase your products in bulk.