They are generally 2 types of online selling platforms.
Type A: One that allow others to sell on it (eBay, Lelong, Rakuten, Mudah.my, Taobao) and ,
Type B: One that sells directly to customers (Amazon, Home Depot).
Note to merchants (sellers) who are offered both platforms from the same company. If you don’t manufacture your own products or owns the brand you sell, it’s a matter of time before your sales is ‘usurped’ by the company because they know exactly what you sell and who your customers are.
That’s what big companies do. They eat the small ones to get bigger.
Read about how Amazon utilizes 3rd party sellers (The Everything Store by Brad Stone) to gain insights on market trends and demands.