In the E-tail arena, two options come to my mind. The first is Amazon and the second is EBay. Yes, there are other options for very small vendors like Etsy or a sole site. There may be 10,000 more sites but none come easily.
Because of its power, Amazon also has greater leeway to curb what them deem to be CRaP (Can’t Realize a Profit) items because of the rise of independent sellers on its site.
They sellers have added hundreds of millions of items to the Amazon site, this helps ensure that Amazon’s virtual shelves are stocked with the variety shoppers expect. And those sales tend to be more profitable for Amazon, which typically collects a 15% cut plus fees for warehousing. Versus EBay’s 8% fee on sold items, you can get some many items listed for free each month. I don’t know what processing fees if any Amazon charges for the credit card processing, but PayPal charges 3%.
But with the option of a sole website attempting to generate sales, these other sites will prosper and dominate.