Selling on amazon, eBay & shopify will definitely increase your sales volume because there are multiple channels. The question is where should you start selling? If you are new I would recommend selling on amazon and eBay to generate data and compare which platform is making you the most sales. I personally make more sales on amazon so I stop selling on eBay and dedicated my full time on amazon. Here is an excel table and graph that I’ve made to compare the 2 channels; eBay & Amazon. As you can see I made a lot more on amazon than eBay
Amazon Selling on amazon is the best marketplace to start your own brand. For example, if you are starting your own private labeling product I would recommend selling on amazon. Amazon is also great for retail arbitrage. Ebay Drop shipping products or getting a quick cheap sale I would go for eBay. Although ebay may be full of sellers from china selling cheap products, that may be a problem if you are dropshipping because their price is much lower or the same as yours, making the profit margin really bad. eBay is one of the best ways to sell your products for liquidation, as eBay customers are mostly shopping for cheap products. eBay is also great for retail arbitrage. Shopify Shopify is the best way to sell your product without a third party such as amazon & eBay as you will have to pay fees per transaction. Shopify will be your own platform so you’re not working for anyone except yourself. This method requires more work because you will need to advertise your shopify store such as buying facebook ads and google adwords. In conclusion, I would recommend selling on both platforms if you are new or just starting this online business. Once you have enough data I would recommend selling on one platform against another, but you can also sell on both it all depends on what suits you.