Multi channel ecommerce on marketplaces is a very cost effective way of producing profitable returns.
For many retailers it is simply too expensive to promote their own website effectively. Pay Per Click campaigns can drive traffic to your site, but unless you have a very strong brand or a genuinely unique offering, or unbeatable price, then most visitors will bounce off.
Search Engine Optimisation is proving to be an increasingly expensive activity with the large Search Engines regularly adjusting search algorithms. Currently best advice is to regularly add new and interesting content to your site, but this is a time consuming and costly business.
The Return on Investment from capital outlay and ongoing marketing, from your own ecommerce website, can be very low. By all means have a website to promote your business, but taking advantage of the global marketplaces will have a more profound effect on your business.