The biggest online retailer in the world has got to be Amazon which was started July 5th 1994 which was nearly 25 years ago now.
Then some other retailers that are pretty big online are Alibaba Group, Ebay, Apple and Shopify.
There's probably some other pretty big online retailers but those are some of the biggest online retailers out there.
Well of course I forgot to mention Walmart but they're both an online retailer and offline retailer as well but they're pretty big as well.