An online marketplace is web application available on the internet. Individual or company creates an account to start selling on these marketplaces.
As a seller competition is very high. All you get from these marketplace is traffic. you are on your own when it comes to marketing your products. All what you can do is to pay for advertising your product to become at the top list of the search result. Fees eats your profit.
To achieve success, your product must be unique and in demand. This way you may consider selling on an online marketplace. Expanding is great for your business.
Other than that consider having you own online store to grow your brand.