What online sales channels should I be selling on?
We recommend starting on Amazon because of all the traffic associated with the platform. You are always more than welcomed to buy products and sell them on your own shopify store. The only problem with that approach is that you are going to need more money upfront in order to run ad campaigns sending traffic to your products. If you are on Amazon then you can optimize your product listing in order to get the most eyes on your product as possible.
We actually wrote a pretty cool article outlining how to rank your products on Amazon. In case you would like to check it out :)
The Ecommerce empire course walks you through exactly how to optimize your Amazon listing and running Amazon Sponsored Product Campaign very effectively. http://ecommerceempire.com/topic/ppc-overview/